Galaxy isn't all that casual friendly.
Rol spoke on this but it has too many cutscenes, a drawn-out intro, and it's probably the most counter-intuitive Mario on the Wii as far as newcomer-friendly goes. Only someone who played and finished 64 or Sunshine would tolerate half the garbage it pulls.
Mario Kart Wii is simple. You race, you win or lose. Just about anyone could get that.
Also, while Galaxy is a fantastic 3D experience, New Super Mario Bros. Wii lends itself much better to newcomers to the series via Super Guide and 4 player cooperative play.
Pixel Art can be fun.