Well, let's compare characteristics to see who has more "character":
Waluigi: No back story, has no family or friends other than Wario (weather they are brothers or not has yet to be confirmed), hates Luigi, is shown to be very crude/vulgar towards his fellow peers, attempted world domination once and is hard working according to a few sources.
That's about it for Waluigi. His motives, place of origin and attitude remain a mystery. But does this mean they should trash Waluigi? Let's look at Mario's characteristcs....during his very first game.
Mario (1981): No back story, has no family or friends other than Pauline, hates Donkey Kong, has a sense of justice and is somewhat of a construction worker.
I know what you may be thinking. "But that's just one year of Mario's life. Waluigi's list spans his entire life time." Yes, that's true, but if Waluigi had as many opportunities as Mario did in the past, mabey he'd grow in character too. Let's look at Mario, after 100 games or so.
Mario: Back story, is shown to have multiple relationships with friends and peers, has a strong sense of justice as well as other traits (happy, determined, brave, caring, strong)...the list goes on.
Now I'm not saying Waluigi should get 100 games; characters don't need 100 games to develope. All they really need is one. In some cases, like Mario, they need a few more. I think that if Waluigi was given a chance, he could at least grow to be admired the same way fans love Wario. But I doubt that's gonna happen any time soon. He came in too late in the game; nothing outside of a DS game or a Wii game could save him now, and I don’t think Nintendo wants to do that.