Splatoon

Matchmaking typically has to balance four factors:

1. Skill level
2. Ping/connection quality
3. Weapon composition
4. Not making you wait in queue too long

During Splatfests, one more parameter has to be added:

1. All your teammates must be on the same team, and prefer opponents on a different team

The more factors have to be considered, the more it's going to have to compromise on some of them. Splatfest teams are a hard requirement that can't be compromised, teams will never be mixed, and that limits the pool of eligible teammates and opponents considerably. So yes, matchmaking quality does suffer as a result.

Plus Splatoon is a very volatile game in general, even when players should be evenly matched it often doesn't look like they are as soon as someone gets a lead and holds onto it. I would conjecture that the system has a harder time than usual accurately measuring player skill levels when matchmaking is inconsistent... which is a bit of a negative feedback loop.

As for Tricolor, a non-mirror needs all three teams, but Team Fame literally didn't exist.