Remind me how this means Tarantula is Grayson's "self-insertion" character?
Sue =/= Self-Insert (at least not automatically). A character who is unequivocably badass, who takes no shit and gets none in return, who routinely makes the regular cast look like chumps and gets no payback, who acts the way her creator wishes he could get away with making the regulers act? Mary Sue/Marty Stu.
Not saying that's the case here (Don't know enough about Grayson's Nightwing run to know one way or the other). But the argument can be made, easily. A character doesn't have to be good to be a Sue.