As a dedede main, I deal with wii fit's ledge projectiles almost exclusively by either jumping over them, or POWERSHIELDING. If you're having trouble with any characters' projectiles, learn to powershield in that MU. Wii fit's projectiles lose a ton of power when powershielded, especially at the ledge.
In fact, I just got off a for glory match with a wii fit that thought ledge camping was a free win. All I needed to do was powershield or jump over their projectiles, and punish their getups (Or lack of.), and it made it super easy for me to win.
Remember though, wii fit's ledge cling is disgustingly low. I don't think many smash attacks reach below the ledge, if any. in most cases you need to commit to punish wii fit, at least in my experience. Just a theory here though, but ness and lucas' projectiles can potentially ruin wii fit with no ledge invincibility, and with less commitment than mario or diddy. their up bs can snake around to hit a wii fit on the ledge, but you'd probably need to time them getting back to the ledge so they would get hit before they can roll on stage and punish you. As for diddy, banana is probably your best friend when youre trying to punish a wii fit. Mario is definitely tricky though, since his projectile probably would help you much with a camping wii fit.
Overall when I play wii fit I'm hunting for patterns on ledgeplay. If you notice one that rolls a lot, then punish accordingly. Same with getup attack and the other getups. A good wii fit will try to make sure that you DONT know what theyll do next, but then again a good wii fit probably wont stay on the ledge if you're great at reading getups. The character does need stage control SOMETIME at least. Bad wii fits though don't know when to quit. Its like cocaine or something.
Any wii fits want to correct or add on to things for me? I kinda just walked into the wii fit boards and thought I'd put my thoughts down so I don't really know if EVERYTHING I said is 100% solid advice but I think I got my point across okay