While it might sound strange to us in the contemporary world where such an address would be considered rude, in the language and custom of the time it was actually a respectful way to address someone.
It is unusual in that Jesus addresses his mother in this rather impersonal (though polite) address. However, this could be simply because mere biological connection to Jesus is of little importance (Matt. 12:46-50, Luke 8:19-21). What is important about Mary is that she is the new Eve; she is the one who says yes to God, whereas the first Eve said no. The first Eve was called “woman” (Gen. 2:22) as is Mary, both at the beginning of Jesus’ ministry (John 2:4) and at the end (John 19:26).
The title is both respectful and theologically important in revealing Mary’s role in salvation history.