Hehehe. A lot of people have been weirded out by this
little fella. In the One Piece thread it even got called the "Chocolate Imposter Plant Dinosaur thingy".
Hmm... they were tying to design beautiful places since BW, but it was the jump to 3D with XY that really allowed them to go crazy, which you can appreciate in the very first route in Kalos. After that all regions have been gorgeous in one way or another, except for SV, which I'd guess was struggling to balance the open world gameplay with the design and all the other development rushes, and even then some places do look great, like the MC's home. Even Galar has some really nice looking cities and the forests really have atmosphere.
The more we find out about what happened with XY the more of a bummer it becomes. If the leaks are to go by, the game supposedly had like 5 cities and 7 or so routes cut out? Not to mention the lack of a third version or DLC to expand on the region. This last one is understandable since apparently Game Freak wanted to take a small break from Pokémon and try making different games before having to return for Generation VII, but still one can't help to lament a bit that Zygarde and all that part of the story were left out for so long.
My guess is that's mostly because the first two are flaws that even fans of the region consider legitimate issues with the games, methinks. The pokédex got somewhat fixed in Platinum, but it's still held as a strike against Diamond and Pearl. I'm guessing the region itself doesn't come up that much mostly because fans talk about the graphics in general, which has never truly been a particularly strong point in Pokémon, especially, ironically after the jump to 3D. I mean, nice looking location asides, a weak art direction overall has been an issue with the games mostly because it mixes the cartoony designs of the Pokemon themselves, the anime looking characters and realistic looking (and many times lacking in detail) textures, effects and places. From what I've seen, the jump to really more varied landscapes didn't really start until BW when they began to try to recreate iconic places from the real world country that served as inspiration. The earlier regions were less impressive, but the use of cartoony sprite artwork, or at most simple 3D models as background backups, made everything look consistent. Let's not kid ourselves, samey routes and settings were a thing in Kanto, Johto and Hoenn as well.
And even then, if you think Sinnoh is bad that's fine, you can criticize it as much as you want, people can argue back and whatnot. What peeves me is how much "overrated" is thrown around in an attempt to shut down positive mouth regarding anything. Yes I know in theory it simply means "it gets more praise/love/recognition than it deserves", but... when exactly do we know something does not deserve any more of those things? Who can dictate that? Why a popular thing is bad but a cult classic is okay to be praised to high heavens? Why something becoming popular is a bad thing (is it wrong for a lot of people to like something)? Oh, turns out that certain popular thing being praised non-stop is okay and some cult classics don't deserve it then? It's incredibly arbitrary when you think about it. Let's not kid ourselves, people really use it as a smug, sneaky way to say "my opinion and appreciation of this thing I don't hold in high regard is what should take precedence, anyone with a more positive take about it is invalid". Look, I'm not accusing you in particular of this, but a lot of randos online don't even have an issue with something but rather with its popularity or just the fact someone might like it, and posture themselves as some scholars of good taste by saying overrated as if that would invalidate anything the game, anime, movie or whatever did right (you can also spot them when they pick legitimate flaws and drill on them to act as if it ruins the complete experience, ignoring everything the discussed game or anything did nail in the head).