I don't really get salt for playing Sonic, personally.
One time though, I picked Sonic and my opponent mumbled "Cancer..." so I proceeded to camp the **** out of him and take the set 2-0.
If you want cancer, I'll give you cancer.
That's pretty much it though. Some people will say things after a set saying that Sonic gives them a tough time or that they don't know the match-up, or even occasionally before a match they'll say it and I'll have already known that I won at that point. Most of the people I encounter usually respect my Sonic though and just ask for advice, which I'll give if you're chill about it.
I don't know, I think it helps if you're at least known a little because if you're just another annoying Sonic that's not really leaving impressions or just live in a scrubby area, then no one's going to really care.
Even had to teach someone how to tell the difference between side/downb mid set once.
I would never. That's not your responsibility. If they ask for advice after the set, then do it if you so choose. Otherwise, don't.
It's not your fault they're bad/didn't take the time to learn. It's not like customs where there's an excuse to ask what it does/how it works.