Is a certificate of cremation a legal document?

A certificate of cremation issued by is a "legal document" to the extent that the court can enter it into evidence as specified in the Federal Rules of Evidence (most State evidence codes mirror the FREs).

Records of a religious nature and similar certificates are admissible over hearsay objection by Fed.R.Evid. 803(11) and 803(12). Records of regularly-conducted business activity are admissible hearsay by Fed.R.Evid. 803(6).