I dunno imo Sonic would have a decent amount of trouble getting past Bowser's wall of hitboxes.
It feels to me like Bowser can push Sonic to the edge pretty easily just by walking into him; fortress out of shield is incredible for obvious reasons, but it isn't his only option. Once he has got enough of a positional advantage on Sonic to force a reaction out of him, dash back > true pivot jab, dash back > true pivot up tilt, retreating SH fair > jab, etc... can cover Sonic so well.
With that said, all it takes is one good read for Sonic to get in and when he does... he gets in. Bowser has no real answer to Sonic's grab and spin mix-ups at lower %s, and no real answer to his Fsmash at higher %s (which will keep hitting him until it sets up for a bair, imo) - after Sonic has momentum, that is.
So +1 Bowser imo, but I've never played the MU. This is all theorycraft so take it with a grain of salt.