It's offensive in this context, as it's meant to be an insult by a non-American. It's like saying "You find it funny that you died from your own rocket because you are an idiot that hails from a country full of idiots." That being said, it's a term that sometimes can be used in jest, for example, if that teammate had instead blown up a large number of enemies with a single rocket, shouting "'Murica! -blam!- yeah!" can be a battle-cry of sorts, as if to say "I'm awesome, and I love explosions, like all Americans!" In either case, it's a stereotype of Americans and as such is offensive to the labeled demographic as a whole. Not all Americans are suicidal idiots who think any and all stupidity is hilarious.