In Mexican slang, it means something like "insolent little douchebag", "absurd fuckhead", or "deliberately, and often arrogantly, uncooperative".
It's highly inaccurate to say it simply means "annoying person", "someone who does something stupid", or even "snob". It can be used to describe an annoying person since an absurd, arrogant asshole will of course be annoying, but annoyance is such a subjective and broad notion and mamón does not come close to encompassing it completely, so it's a flawed definition. Same thing goes for 'doing something stupid' or 'snob', and to these add the fact that there are slang words for each of those that are several times more fitting; like pendejo or fresa, respectively. Although it would be perfectly fitting to call a snob a mamón if he or she has the appropriate attitude, but not a simply someone who does dumb things since pendejo covers that.
- That little mamón thinks he's hot shit with all his little bullshit.
- I used to hang out with him but then he started being a little mamón.
- No seas mamón. No mames, wey.