I wouldn't really call sarcastic Hawke a nice person, more sociopathic then nice, although thats probably the reason the sarcastic personality is my favourite with how funny & inappropriate Hawke can be
Well, it does kinda depend on how dedicated you are to that purple option. I generally shuffle between all three options as I feel appropriate, but just enough of the purple one so that the humorous personality is the predominant one. I rather like that DA doesn't have a real alignment, and you can shuffle it about as you wish when responding to different people and situations. But yeah, full on sarcastic Hawke is a total jerkass, and I admit, I love it XD
The one and only time I've ever really had an issue with sarcastic Hawke is in auto-banter in Mark of the Assassin, because there's one bit of dialogue between Hawke and Isabela that really bugs the hell out of me, when she's complaining about Gamlen being inappropriate.