[2023] How is it a good experience to have a string of predetermined losses?

I would suggest you look into Ranked and Clan Battles. Ranked uses Random Matchmaking, but it functions which an exclusionary system. Each league can only see others that are within the same league (qualifiers see the "next league" as you are auditioning for the part). While this isn't hard-and-fast MMR, it does have a refinement effect which yields higher skilled games on average with each league climbed.

Clan Battles functions in an MMR-like way as you win rating or lose rating through your battles. Your team will eventually find the spot it's going to float in which is indicative of your team's skill relative to the rest.

Brawls Matchmaker does have a basic form of Skill-Based Matchmaker in that it matches off a "# of Wins" in the current Brawl. IE: Someone with 10 Wins and 0 Losses could be matched against someone with 10 Win and 10 Losses because the number of Wins are the same.

As players play more and win more, they can expect to see other players who have put more time into the mode as well.