Toon is just shorthand for character. Mostly found in MMOs (yes, like wow) but fairly common in any game in which one player can have multiple, distinctly varied, characters.
Same thing with aggro. Which is actually a very relevant concept in this game. For instance, despite showing up a red/hostile, coyotes and bighorners have very small aggro ranges.
I wouldn't get too up in arms about it. Unless you find other Internet shorthand ("what u doin?") equally offensive. And in the case of "aggro," I would be hard pressed to find a more clear and succinct way of getting that point across.
Got no problem with aggro. That one at least makes sense.
I guess I'm missing how it is that "character" got shortened to "toon" when it would make more sense to go with, oh say: "char" which is the same amount of characters (hah) and is a hell of a lot more intuitive. Even "avatar" or "avi" or "av" would make more sense than "toon". Oh well, that's just me, maybe.