Well, booing him won't help his self-esteem any
I think of cheering someone as support, too. Not just as some reward for playing well.
I can understand the frustration, but IMO making insults towards these players when they aren't their best isn't really right. Personally I despise the "If you're doing good we love you, if ur not doing good we hate you" attitude that it seems is quite common, especially in Chicago(Sox anyway). Everyone goes through slumps. Ease off a bit on the "eat s***" and "Cocks" and all that other crap, jeez. I realize it's frustrating if not everything is perfect, but that's how it is.