Green’s Dictionary of Slang

(UK juv.) an eccentric.

2001		G. Coughlan Everyday Eng. and Sl. 🌐 Mentaller (a): crazy guy.
2001		OnLine Dict. of Playground Sl. 🌐 mentalist n. someone considered to be a bit lacking in the brain department: i.e. ‘the lights are on but no-one’s home.’. Always more effective when used as part of the phrase, ‘You big spastic, you’re a mentalist.’ [...] mentler n. A mental person.
2012		Guradian 3 Aug. 🌐 What the EDC ravers most recall are the ‘nutbags’ and ‘mentalists’ who flocked to Gatecrasher, the Sheffield club.