Why does this have the "Disgaea 2" tag as opposed to "Disgaea?" In fact, why does the "Disgaea 2" tag exist?

Because there is Disgaea 2 and it has a completely new set of characters? And it has disgaea_2 because Pleinair officially appears in the game, though that makes little sense for standalone pics of her, really.

Pleinair appears in Disgaea and Disgaea 2 (and Phantom Brave), but most pictures of her are not associated with any particular game.
I would say that there's no reason to make a separate tag for Disgaea 2 given how much overlap there would be. Either that or there should be a general "Nippon Ichi" tag which includes anything from the N1SRPG series; it'd basically be like the "Final Fantasy" tag, except there's no official name for the series.

She is more visible in Disgaea 2, she appears in the news report cut scenes

I think in Disgaea 1 she just appears as a sprite character

I didnt know myself that she was in multiple games till i did a search for Disgaea 2 characters after playing the game.

Basically, I think Pleinair is a sort of "mascot" character of Disgaea's lead character designer and, as such, tends to appear in his games as a side-character quire a bit. However, this does mean that unless it's with other characters, the Disgaea tag is unwarranted.

Pleinair is the mascot of Nippon Ichi's most prominent artist, Takehito Harada. Harada has made plenty of pieces involving Pleinair that have nothing to do with NIS.