Fiddler is a TCPMON-like web debugging proxy for the .NET ecosystem. If you want to perform port-forwarding like TCPMON, you can either add a registry entry like this:
HKCU\SOFTWARE\Microsoft\Fiddler\ReverseProxyForPort=target port (DWORD)
or enable Tools/Fiddler Options/Allow remote clients to connect and then add an OnBeforeRequest handler in Rules/Customize Rules:
if (oSession.host.toLowerCase() == "webserver:8888") oSession.host = "webserver:80";