Is it allowed to make jokes about Qiyama, such as saying something like God will hit me on the head on Qiyama for oversleeping and missing my prayers?
It is more respectful to the enormity of the Qiyama to avoid making jokes about it, the same for jokes about Allah.
However, out of His mercy, Allah has veiled certain things from us currently, including the severity of the hereafter, and the extent of His nature, so perhaps Allah would understand that these jokes are due to our ignorance in the material world, in the same way that children sometimes say silly things.
Still, it is better to avoid it.