I have a lot of experience with this match up, and it really all comes down to the Mario knowing the match up or not. The cape = **** against IC (and pretty much everyone else), it will turn around your belay and your squall (very easy for them to cape squall!!!), so it can be problematic getting back on the stage. Never assume that you are safe from the cape, because it can still reflect for a while after the actual cape is gone, AND it doubles (I think) your momentum, so even if you get caped at the end of squall, you will most likely going flying off in the opposite direction. Also don't quote me on this, but I am fairly sure that Mario's down smash comes out as fast as MK's, so spot dodge into d-smash is great for keeping the IC's from grabbing.
Down air separates the IC really well, and once they are separated Mario has a ridiculous (and by ridiculous I mean its just plain stupid how easy it is) time killing Nana unless you act fast. It is also very difficult to camp a Mario that knows what they are doing with fireballs. While its true they don't have great range, and they don't do much damage, they will stop you, and they will force you to make mistakes if you are not smart about avoiding them.
Onto FLUDD! Personally I have never found FLUDD to pose much of a problem, but if aimed right it can royally screw you over on a few occasions. What I have experienced is getting hit by it during a recovering belay, and even though Nana tethers the ledge, Popo just does not fly up to it, and instead drops off. This will most likely not happen to you in your average match, but its still something to look out for. Also, if you see a Mario with a fully charged FLUDD, DO NOT use blizzard near the edge of the stage. If you use it, and the Mario out spaces you, you will get pushed off mid blizzard, and most likely end up de-synced. Nothing boosts a Mario's moral than getting a kill with FLUDD so don't let this happen.
Make sure you keep their moral low by chain grabbing the hell out of them
, because as someone above me said, it is VERY VERY easy to actually perform the CG on Mario once you do manage to get the grab off. Anyway if I had to put numbers on this match up I'd say 55-45 IC against a Mario with little or no IC battle experience, and 60-40 Mario when the Mario player knows the match up.