That may be, but they didn't do horrendously with the graphics. They're really quite good.
Perhaps it's just that Nintendo didn't put as much effort as they could have in the Wii's graphics? Sure, games like The Legend of Zelda: Twilight Princess and Metroid Prime 3: Corruption have great graphics for their styles, but Pokémon Battle Revolution also has good graphics for its own style. It's not like virtual reality, where you'd expect to see the fur on Infernape's back or just how shiny a Blastoise's shell can be. You're not meant to reach into the game and pat your Pokémon on the back for a job well done.
Like you said, it's eye candy, but it's also more than that. Pokémon Battle Revolution can allow you to get items you'd otherwise have to trade for (which in my experience can be far more difficult than putting up with a video game), and there's already been three hidden Pokémon for the Mystery Gift function as well.
I guess my point is simply that the Wii wasn't built for graphics, but rather innovation, so opinions such as yours are expected. Not wrong by any means, but certainly predicted. Pokémon Battle Revolution's graphics aren't meant to be it's strong point, either. Its strong point is getting items you'd otherwise only have one of, and the fact you can battle competitively on the Nintendo WFC without worrying too much about hacked Pokémon. But right there, you mentioned three games that looks amazing. Galaxy, Brawl, RE4, Twilight Princess, hell, even Rayman looks VERY good for Wii games. So being the Wii is no excuse. Quilava's fire looks like cheap cellophane. Grotyle's jaw is so squared, its like, WTF?
Sure, graphics are important, and Pokémon Battle Revolution doesn't disappoint, keeping in mind that the Wii wasn't made for graphics like the PS3 or XBox 360. Imagine if The Legend of Zelda: Twilight Princess was on the PS3. It would look much more like Final Fantasy XII or something, don't you think?