Popplio is a pure Water type, and is one of the three starters you're able to choose from in the Alola Region. Honestly, I actually think of the three Alola starters, Popplio has the worse design of the bunch, although it is still okay, it just doesn't really have as great a stand out factor as say Rowlet & Litten. As well, Popplio is probably the worst of the three to use in battles, again although still okay with stats, and movepool, just doesn't really have great abilities. I still like Popplio at the end of the day, but honestly, out of all the water starters, I actually think this is my least favorite, it just has nothing about it that really stands out with design or battling prowess, and that's basically what got it ranked here.
Absol water starters usually are what I love most in each region, but idk Popplio as cool as it is just I find meh compared to Rowlet & Litten (even though I probably chose Popplio more than I did Litten and Rowlet lol, but that's more just cuz of final evolutions)