Is 8 stone too heavy for a 13.2hh pony?

It's not so much the pony's height as it is his build that will influence the riders weight. A horse shouldn't carry more than 20%of it's own body weight, this includes the rider and tack. But keep in mind that a shorter sturdier horse/pony with thick cannon bones and broad loins will always be stronger and more able to carry heavier weights than taller horses.