This would be beneficial because Fiddler's UI is, in my opinion, a bit clunky. And it doesnt port well to other platforms other than with Mono.
I think Fiddler would greatly benefit from using Qt instead of native Windows because of cross-compatibility and flexibility. It'll also make it easier to target any individual OS with Fiddler's existing soitce code without depending on Mono. I'd imagine this would be something that users would want and designers like myself would dream over, because Fiddler is a very useful and sophisticated tool, but it should have a more streamlined UI to make it easy for any developer, hacker, or tinkerer to build upon.
P.S. If you need it, I wouldn't mind doing design work for the Fiddler project for free, as Fiddler is such a useful free tool and the least I could do is lend a designing hand