It'd be very easy to target him if he were playing for someone like Wigan. Yes, he has the skill, pace, speed of thought and movement to get out of situations where he's got two or three men tightly marking him; but at Barcelona teams find it difficult to have pin him down for an entire game, not just because of his skill, but because that simply frees up space for Xavi, Iniesta, Pedro, Villa et al to inflict damage instead. If he were playing for Wigan, it'd be a lot easier to just try and mark him out of the game. Plus, he wouldn't get the supply he gets at Barcelona - let's not forget they're the best passing side in the world.
On top of that, in realistic/practical terms (not that Messi at Wigan is particularly realistic!) he'd have to adjust to the climate, culture, language, style of play etc.
Any team would be hugely improved with Lionel Messi playing in it, but I don't think he alone could take Wigan to being a top 4 side.