I wouldn't be too keen on doing a lot of agility with a mastiff breed. Jumping isn't really their specialty and while some are very agile, their size is a huge hinderance. (Imagine a Neo trying to balance on a narrow plank.. It's just not what they are meant to do.)
So.. No, I don't think they'd be "good" for agility, not in comparison to the smaller, lighter herding breeds. But, I think doing it just for fun would be great.