I think that it could depend on whether it's a random online match or a 1 on 1 vs friends match. Either way, I don't really find it offensive so much as annoying. For the time it takes me to get a random 2v2 group going sometimes, it's just annoying to have a character run to the side of the stage and taunt all day.
Then again, I can just pick characters like ROB or Pikachu and annoy them by interrupting them until they stop... =P
If I'm playing someone 1 on 1, and one of us pulls off a string of moves or a nice setup for a kill, I don't find taunting that offensive at all. By all means, if I wasn't good enough to see something coming, or I let myself get caught or killed in a 0-death manner, go ahead and taunt. I deserve it. If it's a really close, back and forth game between two players (evenly-matched), then I guess I might find it a little more "offensive." I don't really think that taunting after EVERY kill is really necessary, especially if it wasn't so great in the first place. In that case, it's really more like saying "Z0MG PWNED N00b!!11!!1!shift!1" when you're barely ahead or you were about to be beaten yourself.
If it's "justified," go for it. Otherwise, it can be annoying, but I don't really find it "offensive."