This seems like the best thread to lay out my thoughts.
About Doc: First set vs ESAM, Nairo's Doc looked pretty bad, not gonna lie. But second set Nairo was 100% on point, and essentially did everything a Doc main would have done. I will say however that ESAM going straight into Loser's Finals after losing to ZeRo (which he has went on record to say that he is an extremely mentally draining player to fight against) also attributed to this. Due to this, I don't believe Doc has a winning MU vs Pikachu. The stats and options don't really match up for Doc to have this luxury, but he is a solid choice.
I'm sure Nairo has an awesome Mario, but Nairo likely picked Doc over Mario because Doc's reward on hit suits his playstyle more. Doc has a very inconsistent neutral game while Mario's foundation is solid, but Doc's kill power is some of the best in the game and he can rack up damage pretty easily. So he plays Doc similar to how he would play ZSS. Risky, and read based.
Doc isn't bottom 10 and never has been, but he's definitely bottom 20 and maybe bottom 15. However, Smash 4 is balanced enough that every character in this game has their use, and it's good to remember that.
P.S.: I'm glad ZeRo lost. Not for the novelty, but because he can finally relax. His win streak has honestly been a major stress factor in his life because there was so much riding on him. Being able to pull that mantle off his shoulders will finally allow him to enjoy playing the game again.