Sparkling Area isn't useless in Singles at all. It's a Surf that bypasses Substitute's and will only make you wish you had Surf if your opponent either
A. Has Soundproof which would apply to Liquid Voice Hyper Voice too if Liquid Voice worked like the -ate/Galvanize abilities and isn't available to anything Primarina should be particularly worried for.
B. Is a burned Pokemon that resists both your STABs like Toxapex or Ferrothorn. Why an opponent would chance switching a burned Pokemon that's supsceptible to Primarina's moves over switching in a special wall and clicking Aromatherapy while Primarina's forced out is beyond me. And even if your cleric has been K'Od leaving you no other option, you'll probably take too much damage from the Sparkling Aria to justify the burn heal anyway or run the risk of getting bopped by a different attack. Overall this scenario is stupidly unlikely.
Moving on to Primarina as a whole, I can't help but feel that when it comes to singles, it got shafted the most of the 3 starters. It's just as slow as them but has a HA that's lame in singles (Let's be real people. Torrent ain't best ability in the world but it'll do you more good in more games than having a sound based Surf that doesn't heal burns while still causing damage. Especially since Primarina is so slow and has a bad Def stat.) and a support movepool with almost nothing she can actually exploit without getting outclassed. No Calm Mind, Nasty Plot, speed boosting moves, recovery, etc. in tandem with her low speed and bad defense means she'll struggle to justify herself over other offensive Pokemon in OU, even with that golden typing. Not to mention Tapu Fini will be a constant source of competition. If Liquid Voice just worked like the -ate abilities and if it got more useful support moves, it'd have much more going for it. Will probably be UU at best unless the tutors and events are kind.