Srb2 sprites are at a higher resolution than traditional Sonic sprites, so it makes sense that there would be details that wouldn't be possible at smaller resolutions. I think it would look odd for Sonic not to have teeth in that frame, like he's toothless.
Sonic 2 HD manages to pull it off quite well without looking awkward, and those sprites are at an even higher resolution than SRB2's.
Of course, S2HD's artstyle is based on Japanese promotional Sonic artwork from 1991-1993, whereas SRB2 has a more unique style based on SSN's artwork. I made a quick edit of FSonic's drowning sprite to remove the teeth, and I agree, it looks quite awkward.
However, that's partially due to the odd shape that Sonic's mouth was drawn in. I drew a quick comparison so you could get a better idea of what I mean.
Now, I don't mean to rag on MotorRoach's work here. He is far more skilled than I am, and his work on SRB2 is proof of it. However, I made a few edits to the mouth to make it resemble the classics more, and I think it looks quite nice. This is just a suggestion, of course.
Cue the "who cares, it's just one sprite you see for like one second" comments...
Also I don't think whether SSN would've drawn that detail is relevant, since he forgot the detail of Sonic's whole ears in srb1.
This isn't a good argument at all. Let's pretend SSN actually did draw those awful SRB1 sprites. (Which he did not as already mentioned) Even if he did, that was
twenty years ago. I drew this crappy Sonic (
https://i.imgur.com/p3vMeNn.png) five years ago. I guess that invalidates this one (
https://i.imgur.com/MohXYHD.png) I made last month. Artists are always improving. SSN's actual old art wasn't even all that bad either, and his style is a big part of what makes SRB2 special.