Not to mention, this isn't actually how Amy's ability works. Your directional input doesn't affect which way the hammer goes - she jumps in whatever direction she was already moving in. If you were moving forward, then held back and immediately spin, she wouldn't attack behind her - it doesn't redirect your momentum like that.
Now given that the thok's most critical feature is that it can instantly redirect your momentum... having it only work in whichever direction you were already moving isn't very useful.
One of the reasons for this would be to account for the difference in how turning works with a gamepad vs a mouse. with a mouse you can adjust angles instantly and perfectly with high reliability, with a stick making a short or quick turn is actually comparatively difficult, and the idea that you could attack behind you, turn, and then proceed moving forward, becomes much more difficult on a stick regardless of personal skill, as the ability to effortlessly make fine or extreme movements just isn't there, even with modifiers to effect turn strength like a lot of shooters use with iron sights vs free aiming. Mouse will always beat Pad for this reason and you need to apply control cheats to even the odds. (Look at what quick turns did for console shooters)
I consider myself a very proficient player. I have all but the 2 ranking attack emblems for black hole, giving me 198/200 currently, as well as all emeralds for all 6 characters in clear files, and can comfortably clear all but multiplayer special stage 7 reliably (unless i'm using a character who can circumvent the platform crushers over pits centerpiece of the map with flying or whatnot) all with pad only play on a dual shock 4 and not even using configanalog. There are just things that become a much more severe problem on a pad when you treat it like a mouse. It actually leads to me using thok far less because I can't reliably use it to get onto a platform like i can with a mouse, for instance, as the natural acceleration is much more reliable without having to constantly be in a state of turning. (I use stick AND shoulders for turning, being able to combine both inputs for a faster turn when needed)
One of the most frustrating things is not being able to spindash behind me quickly, despite it feeling like something that should be natural. The idea of releasing the stick to do a different kind of action than holding the stick is actually pretty natural and games like Devil May Cry V (Which I consider myself to also be extremely proficient at) require such things for various attacks which are actually pretty easy and natural. You have to think about the two styles of control differently as they have different strengths and weaknesses. (in Sonic, the subtly of stick movement actually does have alot of advantages, while turning is much weaker and needs subversion to accommodate for these weaknesses. Nights actually is one thing that stick has full superiority over, and any solid runs with keyboard will always be able to be optimized and improved with a really solid ps4 controller, due to the ability to make such fine and natural adjustments)
Here is a very simple and common scenario where the weaknesses of the pad in the current scheme can manifest:
You want to go to the right and then through a door and grab rings, but an enemy is about to hit you from behind and to the right!
(For the sake of argument, that doesn't have to always be a crawla in this hypothetical, it's just what it occurred to me to draw at the time. making it a crab for instance or one of those shell spinny guys from egg rock can also work for problem solving this sort of situation and upping the stress factor involved. We will assume that this is a circumstance where dealing with the enemy before passing through the hallway is the optimal play solution, but that fractions of seconds of delay will increase risk of being hit.)
Imagine this is an underwater segment. Something as simple as wanting to quickly attack an enemy behind you and then proceed through a tunnel becomes more difficult with a pad whereas with a mouse this would be a simple action of turning and thoking behind you and holding S to cancel your momentum to land where the crawla used to be, then turning and thoking again through the doorway to get the rings and leave the room. With a pad, because the camera only thoking becomes a restriction, it ends up being much more reliable to just use normal momentum to hit him and then keeping your angle trained on the door to thok there, which will take more time in an underwater situation and lead to a greater risk of drowning (or in situations where enemies can shoot at you, greater risk of being hit because the thok becomes much less useful for close range use to your immediate sides and back. even if you turn to thok the crawla, you're going to be losing enough time turning to him and then back at the door even if you've some fancy fast turning inputs like I have, that it can cost you intrinsically in ways that you wouldn't with a mouse)
Spindashing behind yourself and then thoking through the door isn't really an option either in the current control scheme, and that's an action that doesn't gain momentum input benefits of any kind from fixed angle aiming. (even trying to roll into the enemy behind you can lead to problems because you have to gain enough momentum before rolling to not trigger a spindash)
Correcting for these fine angles to reduce your momentum again after a directed move is actually not especially difficult with a stick even if the camera is fixed in a single direction, and is something you are going to be doing in alot of games designed around using a stick and relative directional inputs.
Essentially, you trade off aiming strength for player input strength, and coming up with solutions that use inputs to accommodate for reductions in aiming are how you solve the problem of players just not using the thok as much for reliability's sake. The pad absolutely has alot of uses and strengths in even SRB2 and I would strongly push it as having total superiority in the NiGHTS stages, as well as being no slouch in the normal stages, but the current scheme is optimized for the strength of a mouse while lacking creative solutions to accommodate what works best on a stick in all instances.
An addendum to my partial directional thokking input idea that addresses what Prime said, an input that would effect higher level play positively in this way that might work with the control setup of turning on both shoulders and right stick that I use: holding L and R shoulders at the same time to lock directional attacks to the camera to allow for very subtle anticipation of thok control. (Maybe have the zoom pull in by 5% slightly just to add to the feel of fixing your attack directions in one way?) I feel like pushing both together to modify certain inputs (sort of like a soft lock without any form of lock on targeting) could be a pretty natural input scheme to allow for these sorts of maneuvers to maintain.
Edit: so maybe full directional input option, but if you hold both turn buttons the auto face forward stuff and camera based actions go into effect while holding those buttons? (This is assuming the default is turn on right stick and also shoulders) Having it as a forward lock would allow for full range of directional and camera based actions.
(A button, clicking left stick maybe, to snap the camera behind the character direction could also help for quick in the moment momentum adjustments)