Mew is incredibly elusive in the Rainbow Cloud course. It appears as a shimmering, transparent orb that flies around you. To get a clear photo, you need to make it reveal itself.
Throw Pester Balls at the orb. Each hit will make it briefly solidify and change color. After several hits, it will fully reveal itself as Mew. This is your chance to get a perfect shot. Mew will often perform a spin or other unique animation after being hit, so be ready to snap multiple photos for the best pose.