It seems like this idea has evolved into a lot of different threads over the years. Quite a lot of the features requested do exist, so I'm going to mark this as implemented - but if you feel like your specific idea isn't there please feel free to make a new idea.
As a heads up, Thumbs up/down has been replaced by the Heart/Remove symbol across all platforms. Let's address some of the main ideas that have been implemented:
It is now possible to undo Heart or Dislike actions in your Daily Mixes, Discover Weekly and Release Radar. If you Heart a song, simply tap the Heart again to get rid of it.
When tapping the Dislike button, you'll get a choice of 'Don't play this song' or 'Don't play this Artist'. Tapping either will prompt an Undo option at the bottom of the screen for if you hit it by mistake, or your cat walked over your screen (@dredel)
Tapping Heart will save that song to Your Library, so you can head there to check out all the songs you've liked. This will also include Songs and Albums you've saved in other areas of the app. I couldn't find an idea for a playlist specifically of just liked songs (without the rest of the library) so it would be a good suggestion for a new idea.