Totally disagree for some fairly obvious reasons:
1) There is no canon class, gear, appearance, or sex for Shepard, regardless of what the promotional materials suggest. There is no canon Shepard. Sheploo soldier is boring.
2) Mass Effect is as far from a PS3-specific title as it can be, while still being technically a PS3 game. Sony is using characters from their own stable for All-Stars, as is tradition with crossover fighters.
3) Mass effect likes to stay within its own setting, and though the games themelves can be pretty funny at times, the whimsy of a crossover novelty game sort of violates that. Sort of completely.