Can't really say much about Mallow because Tsareena doesn't provide any real benefit to her. It's Lana that I'm a little suspicious of, because if Popplio doesn't evolve, then what's left for her? We saw one of Popplio's bubbles support a Wailmer. If it can support a Wailmer, it can support Lana. Her bubble goal is basically done. So again, what's left for her? Basically nothing except for a capture and maybe Popplio learning a new move, which is what? Two episodes? In what is 99% a three year series, there is no way Lana is getting only two more focus episodes. She has to expand on her goal somehow, and the best way for that to happen is for Popplio to evolve.
Besides, Brionne and Primarina are capable of controlling their bubbles. Popplio is not. That's kind of important if Lana wants to go exploring underwater.
I give you that Mallow doesn't really need Steenee to evolve, however, and there's still mileage they can get out of the restaurant stuff, since unlike Lana, her goal is not all but finished. However, just because it doesn't need to evolve doesn't mean it won't. After all, Zoruagible is correct in that there's really nobody to show off Tsareena because it's Mallow's ace in USM.
To be fair, most anime movesets aren't exactly the most diverse in the world. Snivy knew nothing but Grass moves. Infernape got along just fine with a movepool of Flamethrower/Flare Blitz/Dig/Mach Punch. Three out of four of Hawlucha's moves were Fighting type moves. Greninja knows Cut ffs. I agree that having some Pokemon only know one or two moves is ridiculous, but we shouldn't be expecting diverse movepools when they never have been diverse. And again, stuff like Steenee, Popplio, and Charjabug have pretty shallow movepools until they're fully evolved anyway.