She's technically not Mottled, but she has all of the necessary genes to produce Mottled offspring. She's a Black bird recessively carrying a whole jumble of genes.
If she is from a Jubilee roo over a Black hen she only has 1 copy of the mottling gene, for a bird to fully express the color they must have 2 copies. As she matures she will lose more and more of her "spots" until she only has a few or loses them altogether. That doesn't mean though that the genes are gone... what you need to do is breed her back to either a Mottled rooster or one of her siblings, which would be another bird carrying just 1 copy of the mottling gene. Some of the offspring that she would then produce would fully express the color because they would be "pure" for the mottling gene, carrying 2 copies. They would express the color as chicks, juveniles and into adulthood.
One thing you will have to watch for is red leakage in her plumage from her Jubilee father. If you see any red it will most likely be in her hackle/neck feathers. This will probably become more apparent with age.