My mother went to Ringling Bros' Clown College. Being a circus clown is a lot of work. It doesn't look it, because they're trying to make you laugh rather than to impress you, but the tumbles and folding yourself into a clown car without hurting yourself takes a lot of training. Juggling isn't hard, but it takes a lot of practice. You usually have to learn other circus arts, and when you do them you not only have to do them as well as someone who specializes in them, you have to look funny doing it. Pay attention when the clowns walk the tightrope or get on the trapeze next time you go to the circus; you notice that stuff is hard when the trapeze artists and acrobats do it, when a clown does it in giant floppy shoes while someone sits on their shoulders and juggles it looks like they managed by accident. Also, during shows you have to clean up after the elephants.
Clowns doing birthday parties have it easier, but isn't a very stable source of income. You can make $300 at a party, but helium is expensive, as are costume/makeup. Good clown shoes you can walk in all day will be around $500, a good nose is $20 and you'll be replacing it constantly because kids will always find a way to pull it off and tear it up no matter how much adhesive you use, your wig costs at least $50 and you'll have to keep replacing that too as kids tear out tufts of it. You need to do a lot of parties to cover your costs.