Especially when they look ever so slightly different from the rest of the Slowpoke population. In addition to making sure there's never a shortage of slowpokes that people would ever notice, due to the victims being transformed!
They just use the excuse that they're ALOLA form Slowpoke to explain the difference, and it works every time! Also the victims themselves immediately become confused and get amnesia, so they forget their old lives anyway.~
Link
Keijimatsu
So this is why Team Rocket were so fixated on slowpoke trafficking