When i have time to play, on the days when i play a lot, i catch around 1-2 Mr.Mime per day. Sometimes 3 per day.
I have been visiting different nests where different pokemon spawn.
Talking about it with other players that also catch them at a similar rate, they just spawn at that rate right whereever you are, more when you walk around to catch world spawns, less when you camp out near lure modules. That means they spawn everywhere.
Then a fellow player went to the USA, spent most of the time in new york city, and came home with if around 10 taurus, most of them from new york city, some from other places he also visited.
So do not worry. Go for some glumanda nests or eggsecute nests or whatever. Or just go for catching quantities of any pokemon.
Many other pokemon are much rarer, and many of these also have no nests. I would guess you find around 10 of the regional pokemon before you find another wild lapras or snorlax.