I agree that "too nice", when said by the person, is rarely true.
God, yes. It's the relationships equivalent of saying 'Perfectionism' in a job interview where you're asked what your worst workplace flaw is.
Someone I went out with briefly would definitely say he was 'too nice' and 'too giving'.
Unfortunately, this meant in practice that he had literally nothing else going on in his life apart from our (brand new) relationship still at the casual dating stage and was the human equivalent of the kind of dog that knocks you over with devoted enthusiasm if you so much as glance at it, and fawns all over you, tail wagging, if you walk in the door.
It's kind of sweet in dogs, but needy and desperate in an adult human being who's known you a week and appears to have no other human ties, pastimes, commitments, significant friendships etc.
It's a type -- perhaps it exists in female form, too, but I've only come across it in men. They seem to be waiting in suspended animation for someone to come along and provide them with a life, a circle of friends, dress sense, a routine, weekend rituals etc. They seem to have the idea it's women's job to 'civilise' them, keep them up to the mark and in touch with the rest of the human race, instruct them on clothes, buying furniture etc.
It's really unattractive. I think they must turn into the husbands who are incapable of buying birthday cards for their own parents, or maintaining relationships with their family.