In addition to the commercial, Luigi's also a coward in earlier media such as some cartoons, Nintendo Comics System, Super Mario Adventures, and Super Mario-Kun (in volume 3, Luigi's even portrayed as pathetic in one chapter, in the French version, page 114, and the enemies deliberately ignore him; a Galoomba even says "C'est juste Luigi"; "It's just Luigi"). Official artwork in Famicom Grand Prix II: 3D Hot Rally also portrayed Luigi as
uneasy and
probably clumsy as well. Luigi also has a move in Smash Bros. that has him flailing his arms and a bashful kick as a taunt. Luigi's Mansion gets too much credit for "creating" Luigi's personality when Luigi's already like this. You can say Luigi's Mansion exemplified it, but it's a different matter to say they created this dynamic. Luigi's Mansion may have been among the first games, but I've seen the other media as fill-ins and don't deviate that much from the limited portrayals from the games, especially the early games which had practically no personality dynamics, anyway.
Of course, there is media that somewhat contradicts it, one I can think is Super Mario Bros.: Peach-hime Kyushutsu Dai Sakusen!, where Luigi is a greedy idiot and Super Mario Shirayuki-hime, where Luigi helps defeat Bowser, but there are also games like Paper Mario too where Luigi is just... there, I think.
There is also Luigi being a fool even at an early age, being kidnapped.
