For me, I almost never negrate, unless I really think something is exceptionally dumb, like that guy I screenshotted from the KKK thread. And almost all the MATI stickers I've given have been to people I see basically fedposting, or legit just mad at the internet posting things that would make the site look bad, that kind of thing.
But most of the time the way I see people use the MATI sticker on the site it's literally the opposite of what it's supposed to be used as. It's basically a signal that the post you made, made THEM mad at the internet.
Honestly, a better name for it at this point would be the "I'm not mad, you're mad" sticker. Because that's almost exclusively what I see. It's just a visual indicator that what you said got to them.