there are a few chars that you can't really cape that well in every situation.
Samus - gains height, you can normally do something disgusting of the charged smash variation when she lands.
Dk - turns around, gains a little height but loses no horizontal ground. Dnt do it unless he starts and finished the upB from below the stage, but even that situation the cape isnt a great option.
Bowser - same as DK pretty much, but you can cape the last few frames and gimp his horizontal chance of grabbing the edge.
Sheik - never really tried lol, you're (probably) always better off baiting her recovery onstage -> fair -> something, or ledge drop -> DJ bair her startup frames where she isn't invulnerable.
Fox/Falco side b's - they turn around but instead of being in fast fall special lag like everybody else, they go into some wierd mode where they have to tumble recover. edgehog -> ledge drop -> DJ nair always finishes the job here if a simple edgehog won't.
Falcon - kinda similar to dk, bowser and samus if you cape falcon while he is recovering high, and is in line with the edge of the stage - don't do it, it will give him height. Similarly, if he is trying to sweetspot from below the stage, the cape will gimp him out of his chance to grab the edge.
Ganon - identical to falcon, bu he gains a little more height than falcon. also, if you cape ganon/falcon in the startup frames of their upB's, they gain huge height and for some reason additional in air maneuverability - so never cape those frames (normally mario jumping offstage and caping in midair)
Peach - goes into the same stun fox/falco's side b's produce - you can edgehog her death, but she might upB you after u lose ur invulverability. similarily, you can wait for her to upb again and use a down angled fsmash/dsmash to see if she fails to sweetspot. I'm also sure u can do some silly **** like running off the stage and bairing her into the side of the stage, while she is in her wierd tumble recover lag.
IC's - the cape can hit nana when she is invulnerable (the whole of Ic's upb frames) but it doesnt kill, similarily the cape gives ic's additional height and speed out of their sideB like it does to DK. Not worth it, WD and walk to setup the spacing for a fsmash.
I think that's most of the situations mario can't cape, but if you know what is going to happen before it happens, you can normally get something out of these situations even if u mess up and cape by accident. we cant really cape pika/pichu/jiggs, but i didn't think that needed mentioning
lemme know if anything here doesn't make sense.