That Mr. Mime is definitely legal (that is, entirely within the limits of what is legitimately possible). However, in order for it to be entirely legitimate (that is, not modified at all, obtained entirely through normal gameplay), a chain of unlikely (but still possible) things would have had to happen:
Someone playing a Gen 2 game with the OT "Ash" (unlikely because people usually use all caps in those games) and ID 31121 (just as likely as any other ID, just the case at hand)
"Ash" catches or breeds a Mr. Mime and nicknames it a censored word (unlikely unless it's on purpose)
They train it up to Lv. 50
They trade it to a Red Version (why would they?)
They Transport it into Bank from said Red Version
The lowercase OT and the censored word nickname, while possible, are extremely unlikely, especially the latter. So, while that Pokémon is entirely possible (and, as such, legal), it's too "unlikely", so I would suspect a legal hack.
Question: it has no HOME tracker. Did you zero it out yourself?