They're generally worse, so you might say they're rarely an improvement. It seems suspicious that beginners aren't cautioned from doing such things, but instead told to just throw things at things as a base-line, despite which it usually turns out to be to the detriment of the game. Obviously, if people wished to say something about the given games, then changing the tile-set wouldn't help, just as a reference to other GB games wouldn't necessarily be improved by more colourful tile-sets, but might instead imply roughening them up, although this would rarely be necessary in certain contexts. If you're already changing the dialogue, the locations, the setting, the journey (which in Pokémon games is generally basically the same, anyway, so whatever), perhaps even the sprites and characters, and such, then in all likelihood people wouldn't organically care about the tile-set anyway. It doesn't change the game into a different one.
If you want the tile-set changed, you'd actually want something different, of some form, but you don't want to admit it. This might make it seem questionable. Now, hacks which begin by overloading such areas as tile-sets with shininess without any particularly coherent focus on the game, will tend to therefore not be given much room for actually changing the game-play substantially, other than being quite cramped in gameplay terms while doing so, and as such they will tend to be basically the original game without much coherence or continuity. However, this needn't make the tile-sets unuseable, just problematic. You might want to turn your priorities elsewhere, because realistically people aren't just out there for tile-sets - as if they liked Pokémon in lieu of Dali or Watteau - and if they seem to be then they're lying about what they're really into.
