Steam has … basically, emojis that you can give people, as… basically ‘awards’.

There’s a whole system of ‘points’ that you get when you buy a game or get an in game achievememt, you can spend those points on various flair for your steam profile…

Or, give silly awards to people.

So… I would guess that ‘clown farming’ would be intentionally trying to get as many clown-moji awards as possible.

Why do they do this?

… To be the funniest clown, I suppose.