mr. mime is not preventing you from targeting the bench with cacturne's attack, it just negates the damage part, so mr. mime and cacturne combo really well I've considered them for a league level fun deck.
main reason the combo hasn't really been used competitively is cacturne is a stage 1 grass with 90 hp, so there are few support options for it and 90hp can be koed pretty easy these days.