Trevor finds her Christian piety funny. Some of the funniest passages are those where he argues with his mum about the role of Jesus in their lives. The mischievous child once poo-poos in the living room and the stench produces a community-wide panic that made his mum and grand mum suspect a demon. He finds the whole panic funny. It's easy to theorize that families like his are fertile triggers for a creative existence. Whereas his mum deploys Jesus, Bible and the Church as survival tools to navigate a crazy world, Trevor considers these Christian elements nothing more than placebos.
Dude has a healthy cynicism about the whole Christian enterprise. Again, one may theorize that he is like those kids whose extreme pious upbringing, spiced with enough liberal tension, prepare them for an adult life marked by cynicism or creativity - or in the extreme - Unbelief.
There are more thematic concerns in the book, including racial dynamics in South Africa, the struggles for love, etc. but I'm mostly fascinated by his profiling of his mum.
It's sheer brilliance how he blends the realities of ghetto, with his coming-of-age struggles, with his innocent love-life, with his family drama, against the backdrop of apartheid South Africa, all with his characteristic humor.