Last Will and Testiment

Michigan: Is a will legal if it is signed by another name different than their legal name. That is an alias.

I don't purport to answer in Michigan, BUT generally if someone signs his or her Will -- and that purports to be his or her Will -- using the name he or she is known as, even if it is not the "Legal Name" or the name on a birth certificate, PROVIDED the Will is properly witnessed, and the person was mentally competent and free of undue influence, the Will would be valid.

However, if signed using an alais such as if John Jones signs the Will as "Batman" that might suggest he was not acting voluntarily, or of proper mind, etc. If I sign the Will of Robin Hood, leaving everything to my favorite dog hospital instead of my spouse or kids, and signed it Robin Hood, I doubt that would stand up if challanged.

Wills are reasonably serious and playing games with them makes little sense. You also don't want to encourage litigation.