How is 'Mobu' wasei-eigo?
As mentioned in the comments by F1Krazy already, this information is likely wrong.
The nickname actually originates from his First-name, Shigeo. The Kanji for which can alternatively be read as &quot;Mob&quot; (モブ, Mobu).
This is also confirmed to be the source of his nickname in the Character guidebook as mentioned on the Mob Psycho fandom, a copy of which is included at the bottom of the post.
All though Mobu might not be wasei-eigo for &quot;background character&quot;, it does infer such a character. Some more background on this can be found on Japanese.SE's question: Etymology of モブ