No, he isn't. I've played both characters since the Japanese release, and I'm bound to say that they are on a similar level. Bowser probably is a bit better than Zelda, but they're still not that far apart from each other to say Bowser is MUCH better.
Zelda deals more damage, usually, and she can kill earlier.
Both are comparable, because Zelda has, while no Grab-Release shenanigans, DTilt shenanigans. She also has a reflector, and while it's not a good reflector, it does provide her with a bit of protection from projectile-heavy characters. She is also floaty-ish, and is one of the very few characters that are not or hardly able to be chaingrabbed (thus winning the matchups against chaingrabbers, or being even with them).
Zelda's recovery also is slightly better than Bowser's. And she actually does have an offstage game - a pretty decent one at that -, unlike Bowser, who has to rely on Firebreath for edgeguarding.