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.