I agree with Kisal... it depends on how thick you put the mulch on, how dense it is packed, and what kinds of seeds/ plants you put in there. Some things may be able to push their way through, some not.
But the point of laying down the mulch is to prevent the weed seeds in the soil from sprouting, so clearly it was not helpful for your flower seeds.
We all learn from trial and error and all of us here have killed plenty of plants in our learning process!
