Viewers did care: the teeth got changed before the movie was released. And, I don't know if you missed it, but it wasn't just one niche of the internet commenting on his teeth. The "outrage" went mainstream; even dentists were making hit-pieces on Sonic's teeth. I'm not gonna lie, it was amazing marketing for the movie, intentional or not.
As for "the internet", there will always some small part of it which will obsess and/or laught over anything, doesn't mean they represent anything significant - not even when they're vocal.