I can’t imagine the DNA would still be viable after all this time.
Depending on the environmental conditions around the mammoth, the DNA may be in very good condition. The problem is that it takes more than DNA. In order to get the DNA to function, there must be a slew of other biomolecules present to interpret the instructions in the DNA. Cloning that mammoth might be a bit of a challenge.
I'm thinking of cloning extinct animals, too. I saw a giant bird egg at a museum. If I can get a dried embryo out of that egg, then I may be able to make an incubation chamber using ostrich eggs, and try to make a viable embryo cell... anyway, no sense discussing my whole plan. I don't want anyone else to get the idea of cloning a giant bird before I try it. Or, at least, before I submit a grant proposal.