Primarina has access to encore as well, you know. As well as Psychic, so its not totally walled by Amoonguss and Venusaur. I haven't tested it myself but here are calcs: 252 SpA Life Orb Tapu Koko Thunderbolt vs. 252 HP / 0 SpD Primarina: 143-172 (76.4 - 91.9%) -- guaranteed 2HKO ( This is without Electric Terrain. )
252+ SpA Choice Specs Primarina Moonblast vs. 4 HP / 0 SpD Tapu Koko: 165-195 (113 - 133.5%) -- guaranteed OHKO
252+ SpA Choice Specs Tapu Fini Moonblast vs. 4 HP / 0 SpD Tapu Koko: 136-162 (93.1 - 110.9%) -- 62.5% chance to OHKO
252+ SpA Life Orb Primarina Moonblast vs. 4 HP / 0 SpD Tapu Koko: 142-169 (97.2 - 115.7%) -- 87.5% chance to OHKO
252+ SpA Life Orb Tapu Fini Moonblast vs. 4 HP / 0 SpD Tapu Koko: 118-140 (80.8 - 95.8%) -- guaranteed 2HKO
252+ SpA Choice Specs Primarina Hydro Pump vs. 252 HP / 4 SpD Mawile-Mega: 156-184 (99.3 - 117.1%) -- 93.8% chance to OHKO
252+ SpA Life Orb Primarina Hydro Pump vs. 252 HP / 4 SpD Mawile-Mega: 136-161 (86.6 - 102.5%) -- 18.8% chance to OHKO
252+ SpA Choice Specs Tapu Fini Hydro Pump vs. 252 HP / 4 SpD Mawile-Mega: 129-153 (82.1 - 97.4%) -- guaranteed 2HKO
4 Atk Aerilate Salamence-Mega Double-Edge vs. 252 HP / 0 Def Primarina: 144-169 (77 - 90.3%) -- guaranteed 2HKO
252 Atk Aerilate Salamence-Mega Double-Edge vs. 252 HP / 60 Def Primarina: 157-186 (83.9 - 99.4%) -- guaranteed 2HKO
252+ SpA Choice Specs Primarina Hydro Pump vs. 0 HP / 0 SpD Charizard-Mega-Y in Sun: 134-158 (87.5 - 103.2%) -- 12.5% chance to OHKO
252+ SpA Charizard-Mega-Y Solar Beam vs. 252 HP / 0 SpD Primarina: 156-184 (83.4 - 98.3%) -- guaranteed 2HKO
252+ SpA Primarina Oceanic Operetta vs. 0 HP / 0 SpD Charizard-Mega-Y in Sun: 156-186 (101.9 - 121.5%) -- guaranteed OHKO
252+ SpA Primarina Oceanic Operetta vs. 28 HP / 4 SpD Tapu Lele: 159-187 (106.7 - 125.5%) -- guaranteed OHKO
252+ SpA Primarina Oceanic Operetta vs. 0 HP / 0 SpD Metagross-Mega: 165-195 (106.4 - 125.8%) -- guaranteed OHKO
252+ SpA Primarina Oceanic Operetta vs. 252 HP / 4 SpD Mawile-Mega: 186-219 (118.4 - 139.4%) -- guaranteed OHKO
I mean yeah, but
252 SpA Life Orb Tapu Koko Thunderbolt vs. 252 HP / 0 SpD Primarina in Electric Terrain: 218-257 (116.5 - 137.4%) -- guaranteed OHKO
252 SpA Life Orb Tapu Koko Thunderbolt vs. 252 HP / 4 SpD Tapu Fini: 133-156 (75.1 - 88.1%) -- guaranteed 2HKO
I'll go ahead and be nice and not calc Gigavolt Havoc because spoiler alert: It OHKOes both of them.
I'm not about to say Primarina is totally nicheless, seeing as it does have some neat tools Tapu Fini doesn't have (like Perish Song and Encore), as well as space for anti-Mushroom coverage, but it's incredibly difficult to justify using over Tapu Fini; the tools that Primarina has that Tapu Fini doesn't have are extremely niche, especially considering they are done better by other Pokemon. Still, there is something nice to be said about compressing offensive bulky Water-type + Perish Song/Encore into one Pokemon, and Primarina perhaps got a tad better now that it can use LV Hyper Voice instead of Sparkling Aria (there's a reason ally-targeting moves are seldom used outside of Earthquake), but even so, I'd put it in C- (maybe C if I were feeling really generous)