Pique hasn't been very good in the last few years (except for a part of the 12-13 season, he was brilliant for a while when he first came back from that long foot injury), but he gets even more shit than he deserves.
Because:
1) he's married to a celebrity
2) he does modeling on the side
both of which make him an incredibly easy (and cheap) target,
but most importantly:
3) he's a centre-back... and no matter how funny it might sound to some, him (or any other CB) making a blunder in defense is exactly the same as Alexis, Pedro, Neymar, Messi or whoever else not converting a fair chance, or anything of that sort. If you're a centre-back and you make a mistake and the team concedes, you're a thousand times more criticized than if you're a forward and you don't score. Whereas, in reality, in both cases you're just doing or failing to do your primary job.
Hating on Pique, however understandable it is in some situations and to some extent, is just popular.