Though that might just be the case for most things, I don't think it would be fair to say that for all things. Two traits could develop in conjunction with another, and trait 1 gives an advantage, while trait 2 is a disadvantage. The most in-your-face example I think is the stereotype of nerds and glasses. In short, a good enough trait could get you laid and fed even though you have a crappy trait that came with it.

edit: alternatively, a bad enough trait could completely derail the development of a good train. Maybe dolphins born with 200iq brains develop some other kind of bad trait as a result of the same mutation that doesn't allow for the 200iq trait to permeate.