This topic reminds me of a video that Jim Sterling made waaaaay back in 2014 about Square Enix, in which - amongst other things - he goes off on one about character design, and about how the most iconic video game characters in the industry are ones with simple designs that don't assault your eyes with details. Video is
here if anyone is interested. That's always stuck with me when I think about why the design of something does or does not appeal to me, and I think it applies here when thinking about what people's preconceptions of what a Pokemon should or should not look like.
A lot (but admittedly not all) of the more popular Pokemon, and pretty much all of the first and second generation, have very simple designs that are easy on the eyes and are easy to remember. When you get to things like the Ultra Beasts especially...well. There is a lot of detail in those designs, and they stand out because of it. Whether that's good or bad depends on your perspective, but there is no getting around that they're definitely different in the way they're drawn. Personally I give them a pass because I think that's the whole point - they're from another dimension, so of course there won't be too many similarities between the way they look and the way Pokemon in the main dimension look - but I can understand why people don't like them, looking at it this way. It is harder to like a design when there is no clear focal point for that design. It's the thought "Wait, what am I looking at?" that will make people pause and feel less affection for something, I think. We all like things that are familiar to us, and something like Guzzlord or Xurkitree...well. Yeah.
There also seems to be the notion in some corners that Pokemon are basically fictional versions of real world creatures, making inanimate Pokemon some of the most reviled...but those are actually some of my favourites! Anyone can design something based off a real life creature, as most of the work is already done. But it takes someone with real talent and imagination to breathe life into an inanimate object. Maybe they're not always successful with this - I honestly do not like the Vanillish line - but personally I think they are a lot of the time. You can say "it's just a sparrow" about Pidgey just as easily as you can say "it's just a teacup" about Sinistea, but because you see sparrows every day and your teacup doesn't try to lick your face when you take a sip from it, it's more difficult to relate to the latter design.
Those are my thoughts on the matter, anyway. I have a lot of Pokemon I dislike aesthetically - like every fighting-type Pokemon ever made - but I've never really thought "that's not a Pokemon" about them.