Viola, at this point in the plot, is still under the belief that her only brother Sebastian has died of drowning during their shipwreck. She is still in disguise as a young man and her reply is a figurative and adorned way to say that he/she is the only surviving scion of his/her family.
NO ROMEO AND JULIWET DID NOT SUICICDIDE
Similes compare two things. Because clowns are considered by most to be very funny, the object would also have to be something funny.
Therefore you could say...
A clown is as funny as a medieval joker.
A clown is as funny as a cat barking like a dog.
A clown is as funny as a comedy movie.
