Oli Diddy and Snake are not unbalanced, and Diddy and Snake are not 'incredibly campy'
'Incredibly campy' characters would consist of Oli and Toon Link. Both Diddy and Snake have seen the most success from players who aren't campy (ADHD and Ally respectively). Falco, MK and ICs can also be fairly campy but it depends on the MU and the playstyle of the player, really.
- eg DEHF is a fair bit campier than Bloodcross, who's pretty aggressive. But even DEHF doesn't rely solely on lasers, as you have to use Falco's amazing CQC to succeed (which Larry does)
- ZeRo and M2K are by far the campiest MK players; the japanese MKs and Nairo are all very aggressive
- NAKAT's by far the campiest ICs player; 9B Vinnie and ESAM are much more aggressive
Oli's very very good but he gets hard countered by ICs, solidly beaten by MK, and also goes even with/arguably loses to quite a few characters (Diddy, Marth, Falco, Wario, Peach, TL). He doesn't really shut out any relevant characters except for Pikachu, arguably ZSS, and (depending on your definition of 'relevant') Lucario, DDD, and maybe some others.
Diddy's like Oli in that he's very good but he doesn't have any hard counters, but also doesn't really shut out any relevant characters. He goes close to even with MK, ICs, Oli, Marth, Falco, Snake, ZSS, Wario, Peach, and doesn't really shut out any relevant characters other than Pikachu, Fox, Wolf, and DDD.
Snake gets shut out by Olimar and Pikachu, loses to MK and ICs, and goes about even with Marth, Falco, ZSS, DDD, and maybe Fox. He also doesn't solidly beat any relevant chars outside of Peach, and only arguably wins against Diddy