Some people have a sense of humor that is self-deprecating. Other just have a habit of identifying fault in themselves. Funny or not, it is mostly a very bad idea. Being honest about one’s strengths and weakness is absolutely fine, but putting oneself down is most certainly not. Are you the kind of person who regularly says “I’m such an idiot,” or “I’m just not very good at most things”?
If so, make a conscious effort to cut it out on dates. The other person does not want to keep hearing you saying unflattering things about yourself. Even if you are trying to be funny, best not overdo it on dates.