I'd said Gardevoir out of these two options.
Gardevoir might be a little slower, but it's offensive stats are much better. It's special bulk is also better— Mr. Mime has an horrible HP stat of 40 compared to Gardevoir's 68. 40 HP/120 SpD is inferior to 68 HP/115 SpD. Both have a very similar movepool, but Gardevoir's is tactically superior because of higher firepower. All in one, Mr. Mime is Untiered (PU), hence you shouldn't use it. Gardevoir is mostly used as a revenge killer with Choice Specs/Scarf, so I would give a set for it:
Gardevoir @ Choice Specs
EVs: 252 SpA / 4 SpD / 252 Spe
- Mystical Fire
- Shadow Ball
Moonblast and Psychic are STAB, Mystical Fire hits steel-types like Metagross, Klefki, Steelix, and Magnezone. While Shadow Ball is effective at dealing with Reuniclus, Galarian Slowbro and hits Metagross as well as Doublade harder than Mystical Fire.
However, for the best fairy type in RU, Togekiss and Sylveon are better in other cases.
Togekiss is one of the premier cleric, and possibly the second best (or third) Pokemon in the tier. It has better Speed, as well as physical bulk compared to Sylveon which establishes it as a good cleric and support Pokemon.
Togekiss's typing, reliable recovery, and infamous ability to flinch through checks with Serene Grace Air Slash let it thrive as either a wallbreaker or a utility Pokemon with access to Defog, Heal Bell, and Wish. Its bulk and typing allow it to comfortably check threats like Zarude and Noivern throughout a game, and its recovery helps it outlast its offensive checks, most commonly Electric-types. Various move choices give Togekiss answers to some of its usual counterplay, such as Flamethrower for Steel-types and Heal Bell for Toxic.
Here's a set for it, both for wall-breaker and as a cleric support:
Togekiss @ Heavy-Duty Boots
Ability: Serene Grace
EVs: 248 HP / 8 Def / 252 Spe
- Nasty Plot
- Air Slash
This is a rather weird EV spread from Smogon's sample set, but according to the analysis it is good. The special EV stat investment appears not necessary on it, but the HP EVs allow it to switch-in better with Heavy Duty Boots while making use of its good defensive stats.
Therefore, Gardevoir is better than Mr. Mime but Togekiss is the best Fairy-type to use in Gen 8 RU.
Hope this helps!