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I don't think the art-style is what the complaints are about.
One just expects better assets for the Switch.
For example, the animation work in Pokémon Battle Revolution, a Wii game, is on average much better than most of what one can find in GameFreak's Switch Pokémon games, including the average animation in Legends Arceus, in my opinion.
Sure the Switch games are more content-heavy and due to the open-world nature require more animations (assuming there are quite a few Pokémon included), but they also have much higher budgets and better middle-ware tools (Battle Revolution came out in 2006 on what was then 2001 hardware) to produce higher quality assets.
And when compared to other open-world Switch exclusives the quality of the assets are quite poor.
Take Xenoblade Chronicles 2, a first-year Switch game -- that had average assets as far as Switch games go -- imo, for example. 
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GameFreak either needs to outsource the more technical aspects of their games to another developer (I vote Monolith Soft), needs to take longer to develop games like this (four year development time, minimum -- imo) and/or needs to hire/train their staff better.
