Terrier is 100% correct that after 1928, Ruth rarely put "Babe" in quotation marks but on occasions he would use quotation marks after this date. I once had a 1935 dated Christy Walsh birthday collage made for his son's 16th birthday and Ruth used quotation marks.
Since this envelope was addressed to Mr. "Babe" Ruth, it is very possible he would have used the quotation marks.
I'm not certain the autograph is authentic, but in my opinion, it could be and I would not automatically dismiss it.