Yes, it may be two prizes, but for mimicry you cannot use your own attacks either. Which is bad if they are not set up. So if so, you will be doing 20 damage max if they have only basics. While this happens, they can get set up, candy into a stage 2 and then kill the measely 70 hp thing. Most stage 2, especially exs can easily do 70, considering that you have no previous damage on them.
Aside from that, you can easily use holon mentor, or dual ball to get your Mew Ex onto the feild. Plus if you wanted with a mentor you could get 3, not that you'd need that many. In other words, don't compare Mew shining with Mew star. Mew shining is a good pokemon, but against a Mew EX deck, there's no contest.
To be honest, 90 hp is often just what you needed. Especially considering many pokemon out there that do 70 or 80 damage. Anyways, regardless of that Mew EX is very susceptable to changes in strategy. You can throw out any pokemon on your bench, use it's stage 1 attack (for instance a kirlia DS to heal 30 hp, then next turn use your own Gardevoir EX, and copy the ability to send a fire back to another pokemon before youknow you'll be KOED, there are endless combos, but with both you need the energy, so Mew EX wins b leaps and bounds.