Both Dire Creeper and I are arguing Pikachu is better than our main for a reason. Pikachu isn't actually more popular than weegee. Definitely Mario though, I will give you credit there. But there are at least 4 high caliber Luigis I can think of. For Pikachu there is Axe and DJ Nintendo (only fairly recently).
Pikachu just has more options than the Mario bros. Luigi can only approach by catching people off guard (they don't expect what will be Luigi's choice out of wave dash) and Mario has a horrible time approaching the top characters (at least that's what I think). Pikachu has a fairly safe nair (still not fox level, but pretty good), a very good dash dance, is a fast character on ground and air, and has a small hurtbox.
Pikachu can gimp people fairly safely, where as the Mario bros it's much riskier. Pikachu is very hard to gimp, where as the Mario bros are pretty easy to gimp (god damn needles...). Pikachu's Recovery is insanely good compared to the Mario bros. The big advantage of the Mario bros is their combo game, but if you can't approach well what is the point? Luigi does have stronger aerials than Pikachu, but his lack of aerial movement makes them not as viable (they are amazing combo finishers though, especially the dair).
That's why I think Pikachu is better, but again this is all opinion.