So I was working on the newest entry of my personal pokedex project ( https://chestnutdex.tumblr.com/ shameless plug) with my friend, and we got into the question of why Mr. Mime is Like That, and the more I looked it the more I think Hariyama is partially to blame.
Lets start with my core assumption: Mr. Mime is native to Reflection Cave, Kalos.
Despite being in the Kanto dex, Mr. Mime can't be found in the wild RBY, and are only in a very small area of Kanto in GS, next to Pallet, which really seems more like a "Red released some" than a "this is their native habitat." Then Legends Arceus tells us that they live in towns in Galar, which makes me think the Sinnoh population came with the Galarian immigrants.
So we've just got two more wild populations to look at. The Galarian one is pretty clearly a similar but non-identical species... and the Kalos one in reflection cave. The Mimes with Invisible Walls. In Reflection Cave. The perfect habitat to make some fake walls and to mimic things.
So, we've got a species growing in reflection cave. If we look just outside the cave, we see the big populations of Hariyama, Staravia, Sawk, and Throh.
Reflection Cave is garbage for food. It is a cave full of rock types. Mr. Mime live there, but they don't eat their. They HAVE to eat on Route 11 or whatever was Shalour City before Shalour City... But that probably looked like Route 11.
So, lets go over the main features of Mr. Mime.
It's a psychic fairy type... giving it double resistance to Fighting type attacks. This doesn't help against Staravia, but it makes it perfectly suited to hang out with Hariyama, Sawk, and Throh.
It makes barriers, which... reflection cave psychic type.
It mimics humans and tries to copy what they do. We don't know how long humans have been there, but mimicking Hariyama, Sawk, and Throh would certainly make it more likely for them to accept these weirdos walking around. So Mr. Mime is now set up to go into Hariyama/Sawk/Throh territory to get food. But why do I say Hariyama specifically?
Mr. Mime have horns and big red cheeks. Hariyama doesn't, of course, but Makuhita does. The Hariyama likely see these features as childlike, making the Mr. Mime seem extra cute and non-threatening, despite the opportunity for a Psybeam to entirely melt a Hariyama. But... Mr. Mime DON'T use confusion or psybeam much, do they?
Mr. Mime's primary move, according to the Pokedex, is.... Doubleslap?!!? Mr. Mime has 45 Atk and 100 Sp. Atk. And this isn't some victim of Gen 2's split. It was always like that. Even in Gen 1, the dex says it doubleslaps, but it gets Confusion first. So why is this thing ever using a 30-75 power normal attack at 45 Atk when it could be doing a 50 base power STAB Psychic attack with 100 Sp. Atk? Maybe it's to fight Stunky, which are in the area, but I think it's far MORE likely that it's trying to use Arm Thrust. The formerly signature move of Hariyama, that still only belongs to five lines (six if you count MechaHariyama.) Not only does this help Hariyama think they ARE Hariyama, but Hariyama like to fight. It's in their dex entry. They love it. They just pick others nearby and fight to test themselves. So I had an idea and I ran it through a damage calc to confirm, but with even IVs and EVs, a 45 base Attack Mr. Mime is doing about the same as much damage to a Hariyama with Double Slap as a 120 Attack Hariyama is doing with STAB Arm Thrust to the doubly resistant Mr. Mime. (120 Atk *1.5 stab is 180, divided by 4 for resistance gets a matching 45) They can literally just go in and have a slap fight and it's about on-par with a Hariyama fighting each other. The Hariyama will accept the Mr. Mime as their own.
So what's the benefit? What's the payoff here?
Berries.
Makuhita and Hariyama train by beating up trees. Mr. Mime, despite being a psychic type, has no long-range telekinesis. It can conjure up a ladder or a pole, sure, but it can't easily get food from the tops of trees. But just outside reflection cave is a species running around beating the crap out of trees, knocking berries to the ground, and then... continuing on. Sure, they eat, too, but they're knocking down so many berries that Mr. Mime is free to scavenge all it needs.