Taunting in person is totally fine with me. It's fun, and it's funny. It's fine whenever it's in person, because you can tell the person is just having a good time.
Online, however?
That usually rustles my jimmies. You can usually tell whether or not the person is being cocky or not. For example, they taunt immediately after they launch you from a potential kill. However, your DI game is on point, so you live.
It also bothers me when people taunt multiple times. Like, 3 times in a row is a bit much.
Taunting at the beginning of a match is fun too, it's kind of a "Good luck, have fun" gesture. Unless it's Captain Falcon. That annoys me.
After a few matches with a random person, you can tell if they're taunting to be cocky and say "I'm better than you", or if they're doing it to have fun.
Certain characters taunts bother me too. For example, Link and Captain Falcon's taunts /really/ rustle my jimmies. Then, other taunts like DK's shoulder shrug is perfectly fine.