People underestimate The Joker as nothing but a buffoon, but in reality he's an agent of pure chaos, and able to turn on a dime between schtick and psychosis when need be, or simply just to amuse himself or shock those around him. Likewise if he seriously puts his mind to a scheme he can pull just about anything off.
I mean this is the guy who caught and tortured Lex Luthor, who has frequently outmaneuvered Batman, killed Jason Todd and his mother, crippled Barbara Gordon, tortured Jim Gordon, killed Sara Essen (Gordon's second wife and fellow GCPD cop), stolen Mxyzptlk's power just so he could kill Batman over and over again, without the risk of his being dead forever (and thus ending their 'game') - and without being too concerned that he might destroy all of reality in doing so, tortured and killed the latest version of the Royal Flush Gang, killed reality tampering Alexander Luthor, shot and wounded D!ck Grayson, which lead to the ending of the original Batman & Robin partnership...and those are just off the top of my head.
People who underestimate The Joker as just a clown tend to end up face down, sooner or later.
