You'd still be spending the same amount of effort.
A "kart"maker for SRB2 wouldn't neither make the spriting job any easier nor harder.
You would still have to do around 8 rotations of potentially 33 different frames for a well done job!
In SRB2 a player has different "states", so to speak: Idling, walking, running, jumping, hanging from things, teetering, special ability, hurt, going up, going down... That's all of them, I believe (that takes up to 8 rotations!).
On the other hand, Kart has, what, 7 states? Idle, "running", turning left, turning right, drifting left, drifting right, and hurt, maybe one or two more.
You can recycle a lot of sprites for Kart - Turning left and right can probably be mirrored, same with drifting left and right. The "hurt" sprites can use the same body as the idle ones - just give the face a touch up. That's substantially less work. Ever wondered why you don't need to make such a huge amount of sprites when using kartmaker?
On the other hand, it is harder to recycle frames for a SRB2 character without your character looking "bad" in some aspect, considering you have to pose the character to fit the states of "floating", "running", and so.
Plus uh
[...] cuz THEN I wouldn't have to go around and replace all the sprites like we have to do today.
What do you mean by this? "Replace all the sprites"?
You don't have to replace all sprites ever, unless you made a colossal fuck up somewhere around the start of your character idea.