I read about this idea a few years ago. In order to produce a clone, they will have to use modern Elephants as donors and surrogates. The resulting creature will not be a true Mammoth but a hybrid species (a Mammephant?) and it's therefore a pretty pointless exercise.
Animals already cloned have been found to suffer as a result of defects that have caused, amongst other things, organ failure, joint problems and premature ageing, so these huge beasts are going to suffer for the sake of the scientific curiosity.
To (miss?)quote Jeff Goldbloom in Jurasic Park:
"You did it because you could; without asking if you should."
Man, proud man, drest in a little brief authority, most ignorant of what he’s most assured,
his glassy essence, like an angry ape, plays such fantastic tricks before high heaven,
as makes the angels weep.
William Shakespeare (anticipating George W Bush?)