I think it's a matter of perception and the perspective of the recipient. It matters very little if 10 or 1000 people here think it sounds acceptable in the context you offer. If it is odd to you it says to me that your perspective on such things just happen to be such that mature people don't say things like this and there is nothing wrong with thinking that. There is no rule of law or standard that says what people in a relationship should say to each other and what is or isn't correct or acceptable. If it bothers you then that's as valid as people saying it's normal.

I personally find that terms of endearment and little silly phrases are acceptable and typically welcomed when I am involved with someone.