“Comes into play” is an old term that has since been replaced by “enters the battlefield”. As for power, Button’s Mom’s effectiveness is highly dependent on how common ponies are in the current metagame. If nobody uses ponies then she’s just a 2/2 with hexproof for 3 but if almost all the creatures are ponies (say, if you’re playing Equestria block) then she’s a nearly unblockable 2/2 with hexproof who also paralyzes a creature for only 3. In other words, she seems like a pony hoser to me.