It is also an open question whether Mueller even has that authority. Department of Justice regulations say that a sitting president can’t be indicted. So if Trump just refused to comply with the subpoena, Mueller would have damned little recourse. Trump might even think the blatant defiance was a win…and he might be right.
And Mueller is not a special counsel in the mold of Ken Starr. He’s a temp employee of the Department of Justice. He’s supervised by Rod Rosenstein who would certainly have to sign off on anything as extraordinary as a subpoena issued to the President. If Rosenstein did that, his tenure could be measured in nanoseconds.