50:50.
Pikachu is one of the easiest characters to CG in the game. Once an IC gets a grab, you shouldn't be getting out and you should be dead. However, Pikachu has the speed and the above-par separation/killing nana techniques to deal with it quite well.
WHEN THEY ARE TOGETHER:
In the air:
In the air, Pikachu should generally be winning. Pikachu will generally be in the air, but we should never be directly above them. We should be at a diagonal so that we can fair/T-jolt them. If we send them up, Pikachu definitely has the tools to **** them. Fair, uair, nair, and bair (it separates) **** them. Fair and Bair will be your best bets, just make sure you hit them. If you miss, do anything but land next to them. This is a match that I will definitely QA away. However, you shouldn't QAC that much because if the IC knows what they are doing, they will grab you in your phantom lag. That won't be pretty...
On the Ground:
We flat-out lose. Every single attack we have can be grabbed except for u-tilt and D-tilt only if they are perfectly spaced. Leave the ground game for when you are fighting popo alone/destroying nana.
Separated (Nana still alive)
Obviously, Popo will try to get his ***** back. Most ICs will not be very discreet with this, and just charge at you and hope to get a grab/Dash attack. A lot of the time waiting for them will work. Use fast attacks to keep popo away and then **** nana.
On the ground:
Pikachu has F-tilt, D-tilt, F-smash, and U-smash to keep nana away from popo. If popo is charging at you, use quick attacks. You don't need to own popo at this point. Once nana is dead (another section) we can have more fun. In this part we shouldn't really be on the ground that much.
In the air:
This is where we should be when they are separated. Nair, bair, uair, and dair all **** nana. The constant hitboxes (assuming popo isn't directly under us) can hit popo away too. He can't do much unless he is directly underneath us, so constantly be on the move. Don't get cocky assuming that popo won't hit you. Always be careful. you don't want them to join. Thunder is god at this point btw. If the position is Nana_____Pikachu____Popo you can run towards nana, jump and thunder. Nana will run into it because the AI says to get back to popo. Popo can't do anything about it but maybe shield, and then nana is high in the air, susceptible to another thunder (you will get punished, but its a solo-po punish).
Killing Nana:
One thing about pikachu is that he is the most proficient nana-killer in the game. IF they are on opposite sides of you (Nana___Pikachu____Popo) thunder will destroy her. Assuming she is in the air (she almost always is since all of our separating moves make her airborne) thunder will send her straight up. Since she doesn't momentum cancel and doesn't air-dodge, she will get hit by thunder after thunder. If popo is too far, nana is literally dead.
The main thing in this part of the matchup is to not get too cocky
You can and will get grabbed/sent off the stage if you aren't careful. This puts it back to IC advantage.
Popo Alone
Popo has a CG on us from like 0-40, and it ends with a F-smash. That is some hefty damage, but he can't really do anything other than that. The best strategy would be to just avoid him and do little damage to time him out (assuming we are on same stock nana dead). This is the one part of the match-up where ledge-camping is good. If there is no ledgegrab rule, RUN TO IT. Be careful not to get hit with blizzard and ice blocks, but that is about it. IC will not risk going off the level to hit you since they don't want to die. If they are going to intentionally die so you can't time them out, you are up a stock every time you kill nana. It is a win-win situation for us.
Honestly Popo can't do anything to us. He can kill us at like 140%, but he won't be racking damage because we don't have to be scared. We get hit once...k. Go to the ledge and then wait for him. If we are losing, popo can't do much to get out of our combos. As long as we aren't above him we are good. His other aerials kinda suck.
So yeah, we lost when they are together, but we **** otherwise. We are still the best nana-killer in the game. That is why, IMO, it is 50:50.