Not following the title, not entirely up to speed on the character. But this is what I've managed to piece together as far as what constitutes Tarantula's rap as a Mary Sue:
First, not just the fact that a female character had sex with Nightwing. The fact that author Devin Grayson herself seems to show herself as having a constant fixation on the character. One sees an example of this in the novelization of "Rise of Sin Tzu" which she worked on, and the character of an FBI profiler who is so thoroughly enamored of 'wing that you'd have to have him a) openly drooling and b) openly declaring himself out of the closet to make it any more obvious.
There's also the idea of Tarantula herself somehow managing to one-up most of the Bat family, including somehow getting Batman's consent to operate within Gotham City (I recall her being there at some point, though I am aware Nightwing is a Bludhaven title. I forget where exactly, possible during the War Games arc) despite the fact that he's far less permissive with far more established, possibly more capable characters like Spoiler and Huntress. Anything that disqualifies them for his approval could equally be applied to Tarantula since she was literally gunning for Blockbuster's life.
Again, aware that I may be missing facts and viewpoints here, this is my view from outside the window looking in