"I think all of us know a guy like this. Always bringing up his 'flaws' before anyone else can. Always spinning it in a positive light, or trying to be funny, in order to prove to others that he's "one of the good ones."
In this study on how self-deprecating humor is harmful, it says "self-deprecating humor was much more attractive than other-deprecating humor when used by high-status potential mates, for both male and female raters. When used by low-status potential mates, there was little difference in long-term attractiveness between self- and other-deprecating humor. We theorize that high-status individuals can more easily afford to make fun of themselves."
"Thus, the use of self-deprecating humor by low-status individuals may be counter-productive, suggesting depression, defeatism, subordination, low self-esteem, and/or low mate value. On the other hand, if an individual has achieved high social status, they are unlikely to have truly low conscientiousness, extraversion, or emotional stability, and they must show reasonable agreeableness often enough to make friends and win support. Thus, self-deprecating humor may be a way of transiently faking inferior personally traits, to highlight the discrepancy between the faked traits (e.g., introversion, neuroticism) and the traits actually required to win high status."
Basically, when you're a "low status" person (like a short man), you only come off as pathetic when you self-deprecate.
Bonus shoutout to the guy named "Free_Apples" from where I found this study. In a situation where a tall guy uses a short man's head as an armrest, this uncle tom is focused on the short man's reaction and whether or not the short guy has a complex. Mr. Tom also still tries to excuse self-deprecating height humor, even when the study is right in front of him. A tall guy could literally put his hands on someone and there will be no height complex given. The behavior under scrutiny is the short person's reaction, not the tall person's action. A short man is much more often lectured about having a complex, rather than a tall person being told to not behave rudely around short people in the first place.