It means to be better at something, basically. I think it started getting used in looksmaxxing communities. There it was used in a sense of "who looks better", but often in a... Not sure how to explain it... If you try really hard to look good, operations, skin care, you name it, and then you sit down next to just some random person that never put any effort into their looks but looks better than you, then you are getting mogged. Iirc they took mogging from the movie Zoolander.