Actually, both the Z-Ring and the evolution are pretty undeserved considering Lana is the 3rd wheel battler, if you can even truly call her one, behind Ash and Kiawe. If anything, those two are the truly deserving ones, yet both have been stalled because the writers just now remembered the others exist. Pretty silly if it's evolving only for bigger bubbles...
That's going by the pretence that only battlers deserve Z-Moves, which has never been stated to be the case. Lana deserved the Z-Ring because she managed to tame and work with the most difficult Stoutland, because she selflessly put herself on the line in between a dangerous Garbodor and both her own Popplio and the Stoutland that had not done anything for her whatsoever before. That's why she deserved the Z-Ring. Lana deserved the Z-Crystal because she went out of her way and nursed an injured Wishiwashi expecting nothing in return, and after that went fishing not expecting a reward. She just wanted a fun fishing adventure. And she proved herself a capable fisher by managing to work together with Popplio and reel in Totem Wishiwashi.
For both the Z-Ring and the Z-Crystal, a large portion of what makes Lana properly earn them is her
character strength. Strength comes in different forms and pokémon battles are just one of them. It's also a large reason as to why Lillie deserved her Icium Z. Because she selflessly entered a battle, not expecting a reward or anything, but purely to protect the Sandshrew. It's even something that Kiawe outright says in the second episode. "Only those with feelings for something other than themselves are permitted to use Z-Moves!" To diminish their character strength so readily diminishes a lot of what the Sun and Moon anime is really about at a deeper level, beyond being cute and funny. Pokémon Sun and Moon isn't just about battles, it's more than any pokémon series before it just as much about characters and their choices.
In regards to Popplio evolving. It has a lot of growth in over a hundred episodes. It got consistently and steadily better and better with its balloons, it learned a couple of new moves, it trained and learned how to use Z-Moves, it is very often also seen practising and training in the background in scenes that are often not even about Popplio at all. It has done plenty in its Popplio stage, and evolving is the next stage in Popplio's development and growth.