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I would personally say gens 3-8 all have their problems but a lack of solid designs has never been the issue.
The problem is how the art style has changed.
Sugimori's original style combines cute and cool with a little bit of grit that makes the Pokemon and characters feel more grounded and alive.
In Gen 3, the style shifted towards being just cute and cool, no grit. I actually think that works and looks quite good on the GBA with its super-saturated colors and fairly small resolution.
The issue is that we've essentially still had that exact same design philosophy for the past six gens. If anything, they've doubled down on it. The style looked great in RSE and Pokemon Advanced, but it's totally out of place now that people are playing the games in 3D on giant flatscreen TVs.
If the style had continued to evolve with the games, or if it had just always stayed consistent to gens 1 and 2, I think we'd be in a much better place than we are right now.