Mr Mimes aren't that rare ever since the increase in spawn diversity as well as the recent event that boosted pokestop items and spawn rate, at least from my experience. Before that I'd only caught 2 or 3, now I'm up to 27 just from that week or so.
Plus they're crap at battling so the only ones that I assume would have any considerable value would be 100% IV ones for collection purposes.
But you may as well keep them for now in case we're able to trade useful things like candies or pokecoins and not just trade pokemon for pokemon, I'm sure you've got other things that you could get rid of, like Jynxes and whatnot that would have literally 0 value.
