It's difficult to be optimistic on Pichu's viability in tournament. He has competition with Pikachu, a potential top tier, who seems to have no problem in terms of power. All of Pikachu's weaknesses are set to overdrive in Pichu, so he will have issues with same matchups that Pikachu may struggle in. I don't see him being a very popular pick aside from sandbagging. If anything, playing Pichu can be useful for training to level up your Pikachu game. If Lucina is "Marth with training wheels", Pichu can be "Pikachu with training weights".
I'm not sure about all of that, because pichu is faster, at least vertically, and has less range but more power. So it'll be a different type of playing.
His fair may have ko power, not fully sure. I think bair might too, but not sure either. His fsmash is more powerful than pikachu's, but a lot less range. His f-tilt has surprising range though, and seems quite good. I'm pretty sure pichu's frame data is better in some cases, so that might be in its favor but I dont know how much it is. (dont know if it makes up for less range)
I think they're different enough that even though pikachu will most likely be better (may be top tier too), they're different enough that each may be useful in their own ways. I think Pichu is at least viable, and will take a bit of a different playstyle to some degree, and get closer in. Thunderjolt seems good for damage too, but you can't only spam it since it does self-damage too).
It's hard to tell exactly yet, but I want to see how far pichu can go. It's going to be in my main team. I wont be solely relying on it tho.
(I may have 4-5 mains)