Yeah, Mega Ampharos doesn't particularly benefit from the rain. It doesn't get an ability that is helped by rain, it doesn't get any moves other than Thunder that is assisted by rain (including lack of Water moves), and its Dragon type may resist Water, but it still doesn't want to be taking too many rain boosted Hydro Pumps to the face. The other thing besides taking up your precious Mega slot is that it's so slow that after switching it in, you'll probably be moving second most of the time, which ultimately leads back to the point that if you're going to dedicate this Mega pokemon for any team, it shouldn't be a rain team, but rather a Trick Room team. Agility is really a waste of time and a move slot for something that's already so slow, and there are those with Swift Swim that are infinitely better in that regard, getting the double speed automatically. It's pretty much as perfect as can reasonably be for Trick Room.