Umm yeah...one problem, if a faster user of Prankster comes along and happily Taunts his Whimsicott, then what? Believe me I know what Whimsicott is intended to be used as, but all the same if you don't have perfect Spe IVs, you risk being outsped by other users of Prankster, like the Djinns and such.Whimsicott is not offensive. It uses only moves in the status category (substitute, leech seed, stun spore and other shits come to mind), which Mischievous Heart (its ability) give priority to. Whimsicott needs a lot more defense and hp investments to survive some hits. speeds ivs are rarely significant.
But like I said it depends on what his use of it is, I look at things mainly with a VGC twist in mind, but yeah...if he's just running all priority moves on it, sure the Spe won't matter as much I suppose.