Primarina is pretty good, I don't see why it wouldn't be in OU, except for its Speed, but it could shine in TR, I believe.
First, Sparkling Aria bypasses Substitute, which is a fairly common move. Even a Liquid Voice Hyper Voice will ignore Substitute, if you don't want to get rid of burns that other members of your team may have inflicted.
Primarina also runs a very good Sub + CM set, making it a great Special Tank (backed up by base 126 Special Attack and base 116 Special Defense).
Water/Fairy (in my opinion) is a very solid typing, with only 3 weaknesses, 6 resistances, and one immunity. Water + Fairy also has good neutral coverage, resisted by only 26 Pokémon (though a few aren't in Sword and Shield, like Kartana, Mega Venusaur, and Empoleon).
And Primarina's niche? It is a bulky water type that also has a great offensive presence. With very high Special Defense and Special Attack, it can effectively tank better than most defensive water types.
Hope this helps! :)