Most Whimsicott I've seen are Encore trappers, a role which is almost an instant incapacitation of any Pokemon that does not use a supereffective attack on the switchin. I've never really understood RU Subseeding, especially with the naturally frail Whimsicott, even with Prankster, as it is a naturally defensive role that is fulfilled by an offensive Pokemon. Its only true use is as a sort of revenge killer for stalls, where Whimsi switches in and Subs to block the inevitable status attack.
The Pokemon in general comes across as "nice ability, shame about the Pokemon", as Prankster only really affects speed boosted Pokemon and the occasional Accelgor.
Re: Typhlosion, the problem with using it as a Sunnybeamer is simply that Moltres does it better. Although there is no denying that the Stealth Rock weakness is a problem, Typhlosion also has a problem with it, and that base stat increase from 108 (Typh) to 125 (Tres) gives a Sunnybeam Moltres the best of both worlds - and access to three base-120 attacks at that. Typhlosion's niche is Eruption, which is completely different.