If I open Fiddler and log into 50 different websites from my browser... in a perfect world, would Fiddler ever be able to show me my password in plain text?
If it does, is that a security problem with the website?
If not, how can you tell if a website is sending data unencrypted? Wireshark?
Did you enable HTTPS decryption in Fiddler? If so, then Fiddler is doing something that no web based attacker can easily do, and seeing your password shouldn't worry you.
If you didn't enable HTTPS decryption, or you see your password in a session whose protocol is HTTP, then the website is not sending your password securely, and anyone able to sniff your HTTP traffic could see your password.