This idea has been merged into another idea. To comment or vote on this idea, please visit FID-I-5 Provide a Fiddler for Mac OS X.
I am not sure if it is possible or not but it would be great to be able to use Fiddler on Mac too. Mac users have very few and expensive choices when it comes to proxy tools.
We are working on porting latest Fiddler for MAC. As Eric said this currently does not involve .NET core since Winforms is missing there.
Somebody already proposed this, though, so I am going to merge both ideas.
Fiddler runs atop Mono on Mac today; the problem is the UI framework. In contrast to LinuxMono's port of WinForms, MacMono's port is borderline unusable. Unfortunately, I don't expect there will be any port of WinForms for .NET Core.
Today, your best bet here is to run Fiddler in a Linux or Windows VM on the Mac (or on a different PC altogether) and point the Mac's proxy settings at that instance.