Oh, quite simply that it can evolve, anonymous.
Let me put it this way. Of the 151 Kanto Dex pokémon, several were archived as single-member evolution families. Of those, four are considered to be standalone, bipedal, borderline humanoid or actually humanoid pokémon: magmar, electabuzz, jynx, and mr. mime. HOwever, in the Johto Dex, three of these were recorded with pre-evolutions (magmar with magby, electabuzz with elekid, and jynx with smoochum). Mr. mime, before the archival of the Sinnoh Dex, was thus considered to be an oddity.
Of course, that changed when the Sinnoh Dex was archived, which added magmortar, electivire, and mime jr. (The conditions a mr. mime needs to meet in order to produce a mime jr. are a bit complicated, hence mime jr.’s obscurity up until this point.) Thus, mr. mime was placed in the same general category as jynx, as being one of the aforementioned Kanto pokémon with a pre-evolution but no evolution.
Granted, the argument against this logic is that mr. mime, when bred without the odd incense, only produces more mr. mime, which means it has more in common with snorlax than jynx, but I say what’s stopping snorlax from evolving? There could be an as-of-yet unknown super-snorlax evolution, just waiting for discovery!
That … is genuinely terrifying, Bill. And this is coming from someone who has heard this exact theory of yours so many times she should be immune to its horror by now. —LH
I honestly don’t know what you’re talking about. Who wouldn’t want to see a bigger, more powerful—oh, now I see what you mean. —Bill
The Pokédex According to Bill 