Is it a sin to crossdress?

Nope. It is a sin to wear clothes (men's or women's) in order to avoid going to war or to go to war when it is proscribed for you based on sex. {Doing your duty.} Now a portion of Paul's teachings can be used here, about eating meat and making my brother to stumble. But a man's neighbor wearing a Yves St Laurent dress and lady boots and earrings and makeup is not likely to make his neighbor fall and start crossdressing: he's not going to start dressing because you do. Now cheating on your taxes, that is different. Even if it is a gray area, if another think's it's a sin and you cause him to do the same thing (by example), then you have caused his downfall, you have sinned.

Paul spoke of "to the holy all things are holy, to the profane all things are profane." A thing is amoral on it's own, neither evil nor good. it is only in use that a thing becomes good or evil: if you dress to get your kicks by scaring people or making them mad, then it is a sin. If you dress because it makes you feel better then it is not.