Let me put it this way:
I did NOT notice Waluigi's absence from Mario Kart 7 until long after I had beaten the game. That's how much of an impression he leaves on me.
The only thing I consider him to be a part of is the character select screen.
It's Nintendo's own fault.
They never do anything with the character. They have so many Mario spin-offs where Waluigi could play even the smallest role but ....nope. *shrug*
Waluigi isn't and shouldn't be a Wario Land / WarioWare character.
No offence to his fans but in my opinion Wario does a better job as a solo adventurer, and as for WarioWare, I'm not quite sure, but I don't really see him there. I'm not saying I dislike him entirely, but I don't really feel much for him either.