Trick Room Cresselia usually doesn't invest in Special Attack anyway since it needs all of it's bulk to survive LO Bisharp's Knock Off, Specs Hydreigon Dark Pulse, etc. to make sure it can activate Trick Room ...so that 30 doesn't make much of a difference.
The 5 in Speed means that slower Cresselia will outspeed you during Trick Room, but anything else that benefits from Trick Room underspeeds you anyway since Cresselia is pretty fast for a Trick Room Pokémon with a base 85 Speed so pretty much the only thing you lose here is the "speed-tie" with other Trick Room Cresselia. That nature though could be better in my opinion because the VGC metagame this year is pretty physically oriented, which is why Bold/Relaxed are usually prefered as defensive natures.
It's definitely a keep for a Sassy Cress, but I think you would get more mileage out of a physically bulky one.