To be fair, mainline Pokémon was never on the cutting edge of graphics. When Red and Green came out, the graphics were one of the main complaints, and not even Sprite design but the simplistic overworld. This was in the mid-90s when the Game Boy had nearly died off, the NES was no longer supported by Nintendo, the SNES was past its prime years and had games like F-Zero, Star Fox, DKC, and Yoshi's Island to boast of, the PS1 was out and making polygons the standard, and the N64 was right around the corner. JRPGs also had heavy hitters like Final Fantasy, Chrono Trigger, and Dragon Quest. Then Pokémon comes out and it looks like a game from 10 years prior, but without color and a lower resolution. That's not to say that the 2D graphics in Pokémon couldn't look good, just that the best ones were larger sprites which were at a minimum compared to the repeating tile-based overworld.
Gen VI onward have legitimately high-poly models, but it's hard to appreciate on a tiny 240p screen, and there's nothing interesting really done with the art direction like in certain Wii games like Super Mario Galaxy or Skyward Sword.