Imo, MK/Snake/ZSS are IC's only bad matchups. But the only matchups I can prove with evidence is Peach and Toonlink. So I'd like that to change. Recently, even after several months of me not playing, I show up at this tournament and take out Illmatic and Shadow1pj (sorry guys lolol).
Vs PJ
WF vs Illmatic
GF vs Illmatic
Watch the whole set before you judge. There are a few fluke matches that are dumb.
I believe ICs definitely have the tools to follow Toonlink. You don't have to desync at all, it's all reading jumps. Getting hit by bombs when upairing TL is worth it since TL is taking the upair and bomb damage. If you're good at PSing and know how to use ICs, you should be able to adapt to TL.
And Peach is a bit different because each game is very different. Peach is really good pressuring our shield, but nair, upair, even belay and dair can punish her. Once she has us separated, she can keep control. KOing Nana is very easy. Honestly, this is a very straightup matchup. When she approaches with dair, dash away and hit with bair. If shielding dair, roll away or buffer a nair (if you do it right, nana will only do it and Popo can run away). Her turnips are a useful tool, but not a dominating force. Squall (ICs SideB) is good for a surprise, but if Peach predicts it then nair beats it easily. So when using squall, you have to be unexpected and do it rising so she can't pop out and use nair.
I can go on and on about the Peach matchup, but I believe it's even.