Bitrate & Codec are both available as library filters in Plexamp, so it is already possible to create smart playlists for flac/mp3 etc from within Plexamp
Unfortunately Plexamp currently lacks the create smart collection option from the library filter page but hopefully this will be added in future releases
The other clients will be able to see/use smart collections and playlists based on filters created within Plexamp, but you cannot currently edit those (or otherwise filter a library by those attributes).
That being said, there’s already two existing suggestions asking for Plex to add such filters to the other clients:
Please comment/vote in those threads in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.