People can say mean things in-game, but a lot of is sarcastic or insincere. It can be hard to tell in the heat of battle and with just a line of text. Angry forums posts are more deliberate and truly hate-filled, but every clan has one of those guys.
I've joined many clans' TS servers for strat battles or just for fun if a lot of them were on the battle or siege server. I can't think of any clan that was full of mean people. Everyone, no matter their banner, likes to hang out and chat and have a good time.