Pikachu has a faster run speed making his aerials actually faster, but the overall speed difference is negligible, simply because Pikachu has more options since he isn't hindered by damaging himself with half of his available moveset. If Pichu were not self damaging, it would open up more options for Pichu to be less linear of a character and not punish himself for missing with gambling electric moves, not to mention he just has less range which is also very important for the little range both characters have. Even if Pichu weren't self-damaging, Pichu would still be lower than Pikachu though. On paper and in tournament, Pikachu would just be a stronger character. More knock back on aerials, better back air with a second landing attack on it, more knock back on up smash, and a way to finish people that are high above you after landing it, a faster and damaging UP B for ledge hogs and just better recovery in general since it is harder to follow.
Mario is higher on the tier list even though his range isn't his strong point because he has viable combos, a stronger SHFFL game, his f-smash actually has decent speed and range anyway on top of being slightly disjointed, a reasonable recovery, and overall a decent moveset. Great grab game, great maneuverability, and capable of safe approaching options are what make Mario better than Pichu. Pichu's maneuverability is good, but not as good as Mario in the respect that Pichu has to move very straight forward, which is just easier to read than someone that can approach with a good SHFFL then mix up to a strong ground game which is something Pichu doesn't really have.
Pikachu is just better in every way than Pichu except for landing lag and slightly slower l-cancels. Being a light character that has a limited moveset because of self damaging attacks with little range, full body attacks, less viable edgeguarding abilities like Pikachu's f-tilt and up air spike, an F-smash that can be DIed, a down smash that has seemingly less priority, Pichu is just not up to par with Pikachu. You just have to understand that Pichu's match ups are so limited to compete, they basically have to be Fast Fallers on FD, because every character can just shut Pichu out, including Mewtwo.
All Pichu has going for him is a chain grab on fast fallers that Pikachu does better anyway because his grab attack doesn't damage himself, lol.
