In answer to the original question, yes there are girls that ride trials just the same as there are girls that ride downhill, BMX, road, 4x and hobos round the back of spar.
Why you want to know?
I think there is not so many because you have to be pretty strong to do trials and able to take the knocks when you f**k up which frankly puts me off sometimes as well.
Also trials is not really a mainstream sport and girls aren't really exposed to sport as much as boys so are less likely to even know what trials is.
Another reason is psychological, if a boy sees someone doing something amazing on a trials bike they think 'wow I would love to be able to do that' whereas a girl would more likely think 'I could never do that' this can only be remedied by female rolemodels in the sport and there arn't really many of them.
Sexism of a different kind (not the ribbing kind) has an effect. A few years back my oldest son and his mate were into the idea of motocross so they both got little motocross bikes (KX 80's). His best mate is a right wuss and was really crap, but one day his sister had a go on his bike and was awesome, a real naturel rider, but her father wouldn't let her ride it purely because she was a girl, girls are just not encouraged enough by parents or even sport companies to start anything other than athletics or tennis.
I wouldn't have known what it is if my boyf and kids weren't into it.