Wait, this is about physical appearance?

Shakespeare comes from a time before cameras, obviously. But, not only that, there were no portraits of him painted in his lifetime. And to add to the confusion, there were no physical descriptions made of him during his lifetime. The only information we have on what he looked like come from about a decade after his death. One is an engraving, the other is a (IMO) low quality bust from his funerary monument. In addition, Shakespeare is such a generic-looking guy of his time that there are portraits of other people that were misidentified as being portraits of Shakespeare because they feature slim white guys with goatees in a ruffle collar.

Compare that to Sonic. He's a character that was designed to stand out visually, and one where the company that makes Sonic games is still, to this day, generating new media with photos and videos of him.

So, if an actual portrait of Shakespeare were discovered and shown to Shakespeare experts, I think even then there's a decent chance they'd more easily recognize Sonic. After all, a "Shakespeare expert" isn't an expert on what he looked like. They're an expert on his writing.