Your seed will be fine.....the mowing will actually help agitate it down to the soil surface......Don't mow it super short. Put down enough of the biomass to cover the seed over well but not so much that its smothered to death,,,,It's basically a roof that creates a greenhouse effect underneath it. while still allowing the seedling some room to get through to sunlight....Its best to broadcast clovers after mowing if the thatch is heavy .