Bowser gets ***** by Ice Climbers and smart Metaknights. King Dedede really is NOT that hard of a matchup. I may be just region biased because Dedede's small step chaingrab is banned, but other than that it doesn't get any worse than 60-40. People really overrate it too much. There are much, much more difficult and prevalent problems when using Bowser than the D3 MU.
Some other bad/awkward matchups are Lucario, Pikachu, ZSS & Diddy Kong.
Like I said before, Bowser sucks against an MK that knows what he's doing (unless you're on FD, then you have a shot)
G&W is...meh. It's not that difficult of a match if you just stay put and don't do anything stupid. However, according to A2ZOMG it's impossible on Battlefield and is an overall 30-70 Matchup. Whether you believe that or not is really up to you, lmao.
Olimar is...a pain in the *** for me, but it's not horrifyingly difficult like playing against IC's. It is very frustrating however, just like Diddy Kong.
EDIT: Because I'm bored/still looking for secondaries/etc:
Do you guys think any character has a somewhat similar playstyle to Bowser? I mean, obviously no one relies on grab releases, but I mean like tilts/a defensive shield campy playstyle <_<
I'm currently working on G&W so he's put; but besides that I have no one else. I WAS working on Marth because he is one of, if not THE the best secondary for Bowser...
Thanks for answering my question if you do ._.