Is it possible to have a mob generate an ore in the chunk it's killed in?

make a custom feature mod element of type ore and place it in the location where the creeper died upon death using procedure attached to death event in the entity triggers tab

or if you mean you want the ore to spawn underground, use an indexed loop to loop while decreasing y position index until the block at the y position index is equal to stone or whatever, then go down a few more blocks and place the ore