It's half a simile, as in Ashmann's explanantion: "cute as a button". The general meaning is "very cute indeed", but the comparison is dropped to make the listener imagine what the thing is cute as. You'd use this in a slangy casual conversation, but not in regular daily speech. This series ran on TV a while ago: www.youtube.com/watch?v=606eK4abteQ The main character is "beached az", which implies "beached as a whale"... errr, but he is a whale.

Language changes over time and so nowadays teenagers say this a lot

yes it is.. it all started with the saying "cute as a button" (dont ask me why a button)...just meant very cute, then got shortened to "cute as", then they started with other words, such as "stupid as" "sexy as" etc etc...