I've always found Nintendo's obsession with stuffing Mario into every damn game they make to be stupid. It's not like he's even especially likeable.

Also I wouldn't say that Crash Bandicoot was the PlayStation's mascot. He was one of them, sure, but there was also Lara Croft, Solid Snake, Abe, Cloud, Spyro, Gex, etc. etc.

Xbox 360 doesn't need a mascot unlike Nintendo's consoles. Nintendo's games are usually light-hearted party games that you can pick up and play and do so with a bunch of friends round your house. The 360 (and the PS3, really) is more of a serious console with deeper and more challenging games. It just doesn't suit party games like kart racing - that wouldn't be anywhere near as fun if you played it against faceless internet strangers.