The only problem I see with games like Modnation Racers, is they have an identity problem. And Sony knew this with LBP which is why they tried really hard to push the "sackboy" identity. But gamers identify games by their main characters or something in the game that is "tangible". That's a real issue for "creation" games because there is no central character, central theme, or central setting that mainstream gamers can grasp onto. It also makes marketing the title rather difficult.
So although it does have that similarity to Mario Kart, the lack of a true identity could hold it back. People go in an ask for a Mario game or look for that red plumber. That's something that will hold this game back unless an extremely effective marketing campaign can be drawn out by Sony.
I don't doubt for a second the loyal Sony userbase (which by sales has shown to be a very consistent buying userbase) will probably buy the title in masses, but getting that mainstream support could hold it back to being more successful than other new IPs.
