Fixed: Could not use Times New Roman in Safari
Screenshot says it all... websites using Times New Roman looked horrible (and my collaborators like using that typeface for Google Docs, so it was important to me). This was new behavior with macOS Monterey.
I eventually figured out that I had two copies of Times New Roman installed. Opened FontBook.app, clicked on "User", scrolled down to Times New Roman, right-clicked to "Disable", rebooted. (Not sure if the reboot was necessary, but what the heck.)
Things work as they're supposed to now. I hope this helps someone spend less time solving it than I did!
MacBook Pro 14″, macOS 12.1