I don't think you have to be a JW to be "too nice." My grandparents were far too nice, but they wanted nothing to do with JW's, bless them! I consider myself almost "terminally" polite, but I'm working on a more confident, in-your-face attitude.
In fact, I'm pretty sure I was much nicer before I was a JW. I was brought up by my grandparents until age 11, and learned acceptance and tolerance as well as kindness and generosity, qualities that are not well received in the Watchtower world. In fact, this realization was one of the reasons I finally got out of the cult.