The description is a little vague, and I know that I get cards for 'playing' it even if I don't actually keep clicking monsters, but other than the cards is there any reward for playing the Steam Monster Clicker game?
You indeed get some cards by doing it.
You also get a 100xp-worth badge, for your steam profil, once you've reached the level 120. Note that you level up even when you do not play the game.
EDIT: After being level 220, the badge you got at level 120 is improved. This badge gives 125xp instead of 100.
It also helps the community to complete the objectives which put some games on sale. For example, the first objective unlocked a -50% on the game Crysis 2.