Does Mob appear in One Punch Man? I was reading One Punch Man chapter #100 and in the cover there is a guy who looks like Mob from Mob Psycho 100. Is it him?
Just wanted to let you know that that is not the cover for the 100th chapter but rather for Chapter 65 titled Sisters. 

Regardless, those are indeed characters from Mob Psycho 100. Starting from the left, we have Dimple hovering above some people then Mob and Tome (president of Telepathy club). 

As a side note, from what I could translate from the description above Mob and Tome, the TV アニメ ... モブ サイコ 100 ...東京 MX... means TV Anime ... Mob Psycho 100 ... Tokyo MX. This chapter might have been released before the start of broadcast of the Mob Psycho 100 anime and the cameos of characters, together with the description, was probably meant as an announcement to the new anime that was to be broadcasted on Tokyo MX.

This does not mean however that Mob appears in OPM or is set in the OPM universe. That is only a cover photo, not an actual scene in the manga story.