Is the saying "Those who pray have finished; people have stopped praying" considered mockery of religion, and if so, what is the expiation for mockery?

The aforementioned statement does not contain mockery of religion, for mockery is to belittle and disdain, and this is not present in what was mentioned. If your statement, "people stopped praying," means that the congregants finished praying in the mosque, then there is no harm in that. One should be wary of yielding to whispers (waswas).