For a serious answer to this question - I don't think it really is but I do think it's misunderstood and underrated. The main concept of the game isn't really like anything else I've played.
The localisation in questionable, the story is filled with cryptic keywords, the story missions are for the most part not great (except for the final boss)...
But the core game is pretty novel and a lot of fun. Only really lacking in variety. It's not even like it's "not a fighting game" - I mean, it's definitely not one in the traditional sense but it does have 1 on 1 combat sometimes with actual fighting game combos and stuff still.
It reminds me of Brutal Legend in both gameplay and reputation. The gameplay similarities aren't too deep (but there are quite a few, hell, even the aforementioned lack of variety is a shared issue) but I think both games have gotten a bit of a similar unfortunate mixed reception - because of its differences with the rest of the series in GG2's case and because of differences in EA's marketing vs. the actual game in Brutal Legend's case.
It is pretty funny that so much lore that sets the stage for GGXrd and Strive is tucked away in GG2 though.