Alright, so imo, Mew should be A Rank because of a few efficient reasons (most of which Boorego covered; I'm just adding in my own opinions to them):
It has a completely massive support movepool, containing moves such as Fake Out, Thunder Wave, Will O' Wisp, After You, Trick Room, Taunt, Gravity, Hypnosis, Dual Screens, Tailwind, Icy Wind, Role Play, Skill Swap, the ability to set any of the 4 weathers manually, Safeguard, Snarl and Swagger. Nothing else in the metagame has all of these support options. At the same time, it has the biggest offensive movepool in existence as well. I've recently been looking into a moveset of Psyshock, Fire Blast, Ice Beam, and Protect with an Expert Belt. It shares Cresselia's great super-effective coverage, but with higher BP coverage options, and a higher 100 base SpA. That said, don't expect it to sweep teams, as bulkier threats will usually be knocked down into easy KO range; it generally is just good at softening bulkier threats up for the rest of the team to take out. Mew's typical set that I have seen is Fake Out, Taunt, Will O' Wisp, Filler (sometimes Zen Headbutt, sometimes Gravity, sometimes Tailwind, this slot is always changing). This set capitalizes on Mew's faster Fake Out than Hitmontop, allowing you to stop faster Follow Me users like Togekiss when Top couldn't. It also is great for shutting things down with Taunt, and WoW allows you to cripple physical attackers. Let's be honest here, Mew is glue. You can tailor make its moveset to fit any of your team's needs. Sure, some roles are outclassed, but a ton aren't, such as "Fake Out Lead", "After You Support", etc. Being the only Psychic-type with Snarl is also a pretty big boon too.
From the performances I've seen from it, I would judge it should be A, because you never know exactly what to ext pect, and because its 100/100/100 bulk makes it hard to take down without Ttar, Hydreigon, or others. The additional facts that Mew makes for one of the best Trick Room supporters in existence (ty After You), and that Mew's offensive capabilities are also sufficient, since it has a pseudo-Thunder Wave immunity in Synchronize, make it a very capable Pokemon on the battlefield.
As a final note:
Can you be more specific? Have you actually used Mew to back up what you're saying?
Here are the utility moves that Mew learn that Cresselia doesn't: After You, Taunt, Will-O-Wisp, Snarl, Heal Bell, and Tailwind. I guess if you want to make the most out of Mew, you would be taking advantage of Will-O-Wisp, Snarl, Tailwind, and / or Taunt, but I still don't find it all that appealing.
I think we're going a bit overboard here (I also think you are assuming too much, Mew has advantages in attack power and speed over Cress in base stats alone, before we even get to the movepools), Mew can use options Cresselia has WITH options it doesn't. For example, a set like Fake Out, Taunt, WoW, and Sunny Day could efficiently handle various weather teams, especially Sand, which would despise WoW. In terms of usage, I have used After You Mew, and I can confirm that it is amazing at what is does. While I haven't used support Mew, I have played against it various times and can easily back how difficult it is to work around (Fire Gem Overheat off of Rotom-H didn't kill it, and in the meanwhile it WoW'd my poor Hitmontop, putting me in a difficult spot).
So, my summarized conclusion, I am
still going for Mew for A.