"WhAt KiNd Of TeAcHEr Is A CHilD's GrEAteST FeAr?"
Neville literally grinning because he knows it's silly. Absolutely no one, not even Lupin, a Professor, shows alarm at this. Also, Neville literally says he doesn't want it to become his grandmother either.
Harry tells Lupin that his Boggart would have been a Dementor. Lupin's response is not, "oh you must really be afraid of dementors." Nope, he says that it suggest his greatest fear is fear itself.
Now that we have established that the Boggart's form can represent something instead the thing actually being what you most fear, let's look at what Hermione's greatest fear is:
That's right, McGonagall. But WhAt KiNd Of TeAcHEr Is A CHilD's GrEAteST FeAr? Her greatest fear is obviously not McGonagall but failure. In addition, Lupin's Boggart was the full moon and he is obviously not afraid of the fucking moon, which is a lifeless rock floating in space, but what it represents to him; the turning, the pain, the loss of control.
Neville is nervous, Snape makes him more nervous with his assholery. I would say Neville's fear is also failure, especially failure to live up to his parents that's been instilled into him by his Grandma (whom Neville also didn't want the Boggart to turn into).
But how about Bellatrix you say? Shouldn't Neville's greatest fear be the woman who tortured his parents to insanity? Maybe, but people, especially kids aren't logical, particularly when it comes to things we fear. (See: Ron's greatest fear is spiders even though his sister was possessed and almost killed by Voldemort last year.)
Don't get me wrong, Snape is absolutely a dick to Neville, but y'all be blowing this boggart thing out of proportion. Learn to read critically and stop making shit up. God.