Resident Camomons Meteorologist, Smogon University
- - Cresselia turned out to be pretty underwhelming. It's as bulky as always, but super passive 'mons in general are extremely easy to punish in this meta with stuff like Bisharp and Sciz running around. On offence, it's outclassed as a Healing Wish / Lunar Dance user by Latias, Celebi, Alo, and even Clef due to their more helpful defensive typings, versatility, and offensive presence. As a CM user, again, Clef is just head and shoulders above Cress, due to its better Ability, typing, and ridiculous movepool. Not inherently bad, but competition + meta trends make her really hard to justify at the moment.
Not to mention Necrozma being a rather superior option for a defensive Pokemon thanks to Prism Armor and access to Stealth Rock (and Brick Break to lure Bisharp out; otherwise, Necrozma's movepool is way too odd).
I have two very successful teams that feature Necrozma (the first one being unusable thanks to the Gyarados ban), and because of my tendency to run balance most of the time, I find Necrozma being a good Pokemon to use on such archetypes primarily due to Stealth Rock and Prism Armor. Plus, Necrozma isn't that passive unlike Cresselia; its Special Attack is nice enough to deal damage even uninvested to a point that Psychic becomes a good choice as its STAB attack.