The worst SRB2 level I've ever played has got to be the 1998-1999 era Deep Sea Zone Act 2 (the one in "Beta Quest" in SRB2 The Past). It's an incomprehensible maze of similar-looking rooms covered in near-total darkness. It manages to feel even more claustrophobic and maze-like than Mine Maze, and it's even less fun. At least Mine Maze has some cool setpieces. Similarly, the DSZ Act 1 of the same era has some rooms that are somewhat memorable (if only because they remain even in the 2.2 versions of the levels), but Act 2 just has all the problems of Act 1 and none of the decent stuff.
It's kinda cheating to pick a level like that, though, since it's just an extremely unfinished prototype that wasn't intended to be released as-is. It's like rating a bowl of cake batter someone left on the kitchen counter as if it were a finished cake.
That said, if you ignore the historical context for a moment and view the level as-is, I think Act 2 is quite useful as an example of how not to design an SRB2 level. It's not ultra-difficult, nor is it comically easy. But it is just hard enough and complex enough to require you to pay attention, while also being too confusing to comprehend and too bland to be interesting. And even if you know exactly where to go, the platforming feels clunky and unsuited to the camera and controls. Some levels become more fun as you get better at them, but this level just goes from mostly confusing to mostly boring.
As for what could be done to improve the level... well, there's really nothing to say, because that problem was already solved over a decade ago with 2.0 Deep Sea... and then solved better in 2.2.