A good number of the carnival's attractions appear to be there on less than consensual circumstances, which is an environment that Mr. Big Top personally cultivates. He swindles unsuspecting patrons into binding contracts, leaving them caged indefinitely.
He puts on an air of grandfatherly showmanship, but he's as monstrous inside as his slimy appearance would suggest.
Marvel Comics continuity
Mr. Big Top has run the Cosmic Carnival as long as anyone can remember. A number of years ago, he abducted a human outcast named Berko, and promoted him to boss of the sideshow when the grousing bald man began to do tricks for the carnival's patrons. Later, Mr. Big Top signed on a group of human children and the Autobot Sky Lynx when the latter brought the children onboard without any form of payment.
Sky Lynx's resulting fame caught the attention of Optimus Prime and a group of travelling Autobots. When the Autobots boarded and asked if Sky Lynx and the children could be released, Mr. Big Top cited their binding contracts and gave Optimus Prime and his companion Goldbug free passes. Optimus and Goldbug tried to leave with Sky Lynx and the children anyway, and with the help of Berko, they escaped, leaving Mr. Big Top trapped in one of his own sideshow cages.
This did not make him happy, but the patrons seem to think he looks kinda neat, if ugly.