A rip-off? Well perhaps it is too early to tell if it is an inferior product but a clone yes, as the game is far to similar to Smash Bros at the moment to be called anything else. It isn't like say Jump Super/Ultimate Stars which has some fundamental gameplay differences despite the general idea being similar. That said I can assume the other main reason why people are so sour about it may also have to do with the fact that Sony doesn't have much attachment to people as Nintendo does. Its characters aren't as "beloved" as Nintendo's, and thus makes this seem less impressive from the start.
So this game will have to do a lot to impress and get past that stigma, but don't expect it to. I felt Blur was a far better game than Mario Kart will ever be but quite often I hear people refer to it dismissively as "Mario Kart with licensed cars" and nothing more.