Isn't Piplup really popular in Japan? (Probably thanks to the anime.)
Idk, when 6th gen came out everyone thought Froakie was the bomb or at least my friends did and at the afterschool program I used to work at Oshawott was the favorite.
Though, I don't think it's so much that people don't like the water starters as much as it is that people are less enthusiastic about the starters looks after third gen and in some cases fourth gen. Personally I lost interest in the starters after 4th gen. I never really used any of the 5th gen starters when I played Black 2 and I traded my Kalos starter as soon as I could. The looks just didn't do it for me.
I heard Piplup is popular there yeah, but I'm only talking about people's initial reactions and not when we're given time to be exposed to them through the anime and whatnot, and I'm also only talking about the water starters post gen 4 because like I said, I wasn't around during the pre-release hype for that generation, so I have no idea who the favourites were initially, anywhere in the world. (Side note: I hated Dawn's Piplup, but Piplup as a Pokemon and just design-wise is cool).
At least in the English-speaking world, Oshawott was despised for sure when the gen 5 starters were released, but ofc there were exceptions (aka the people at your program, myself to an extent, etc.) (Another side note: contrary to my opinion on Dawn's Piplup, I loved Ash's Oshawott and it made me like it a lot more as a Pokemon). You could be right about Froakie, the gen 6 starters were definitely a lot more balanced in popularity, but I recall that at least on Serebii (if I'm not mistaken) it was the least popular and it's "white glasses" and "frubbles" were made fun of a lot on the internet I think. I guess Chespin and Froakie kind of shared similar amounts of dislike.
And no I agree, the dislike has nothing to do with them being the water starters, it's of course their designs. Maybe you're right that the designs of the starters overall since gen 4 have had people less enthusiastic about them, but individual starters past gen 4 have still been plenty popular, like Snivy, Fennekin, and now Rowlet. It's simply coincidence honestly.