Hmm, I dunno. Screens from Luigi's Mansion on GC look better than Dark Moon's, but then again through 3DS experience you'd have to remember:
1.) The GC's game's graphics were very, very detailed even for a Gamecube launch title. Likely more of a result of years of work prior to launch than relating to the system's horsepower.
2.) 3DS always has sharper textures to accomodate for lack of detail, and yet from actual playability Dark Moon could end up looking much better than 2D screens show. GC screens still have that "blur" from videotaping or lack of progressive scan.
3.) Seeing how a 3DS game looks depends on what's in the eye of the beholder. I find that with my 20-15 vision I end up admiring a lot of what I see, but it's mainly for games that really pull off the 3D effect nicely (like Starfox 64 or Mario Kart 7) in my opinion. I also notice that some games look better to certain players' perceptions than others', like myself.
4.) I'll always end up arguing or debating over a game's visuals or a game in general on a game forum. Do yourself a favor and don't take my word for it, just download the Dark Moon videos off eShop, then decide whether to buy or try the inevitable demo because you'll get the most out of playing it anyway. Cheers.