It's a false stereotype. At Oxford, every college enjoys dissing every other college, and some colleges have particular reps attached to them. Each tale grows in the telling.
For example, everyone at Oriel is supposed to be a Tory, everyone at Christchurch is supposed to be an Etonian whose family owns Warwickshire, everyone at Wadham is supposed to have blue hair, facial piercings, a fake northern accent, and ninety seven gender identities, everyone at Worcester is supposed to be a Maoist, everyone at St Peter's is supposed to be drunk or stoned 24/7, everyone at Balliol is supposed to have three brains, everyone at Queen's is supposed to be a Northern Chemist, and so on, and so on. It's mainly just banter, although some of the stereotypes can to some extent become self -fulfilling prophecies. Merton is as much fun as any other college, which is to say, plenty of fun.