You're making a contradiction here. If Sylveon isn't comparable to Mega Gardevoir, then why do you mention Mega Gardevoir's support options? If you mean to say that moves like those you mentioned support Gardevoir in her offensive role, allow me to remind you of this: Xerneas can't utilize these moves, but it also doesn't need them. What it needs is Geomancy, and in some cases, Substitute. Gardevoir, on the other hand, needs them and can't even use them well enough. First of all, it's frail by Ubers standards, at least physically, and with 80 Speed, it means that in the turn it Mega Evolves it's very vulnerable. To go with this, even after it Mega Evolves, its utility is very hard to use properly. Where are you gonna cast Will-o'-Wisp? On Extreme Killer Arceus?
252+ Atk Life Orb Arceus Shadow Claw vs. 0 HP / 0 Def Mega Gardevoir: 294-346 (106.1 - 124.9%) -- guaranteed OHKO
Gardevoir simply lacks the defense needed to use Will-o'-Wisp. Taunt is another of her options, but that, too, leaves her wide open, and unless it faces something like defensive Dialga, it won't do it much good. Destiny Bond is perhaps the best move it has among its surprise movepool, but Mega Gengar does that way better. And remember that Mega Gardevoir also takes up the Mega slot.
To summarise it: Mega Gardevoir and Clefable are viable in Ubers, yes. But they aren't what you'd call good in any way. They do have their usage, yes, and they do have their niche. But don't forget this is the Ubers tier. Many Pokemon have a niche, but it doesn't mean they are good. C Rank is good enough for Mega Gardevoir and Clefable. This is C Rank's definition:
Reserved for Pokemon with viable offensive or defensive capability. They have certain niches or utility that allow them to perform in specific roles. Reliant on fairly large amounts of team support to function and minimize their inherent flaws. They may also suffer significantly from opportunity cost to the point of being mostly outclassed.
I think it describes Gardy and Clefable perfectly.