I can see where Catholics could see this verse as a universal statement of the Motherhood of Mary. But I believe it was specific in time to John. Otherwise, this universal principal would have been mentioned elsewhere in the New Testament - but it is nowhere to be found in the teachings of Paul, Peter, or John.
It is mentioned, in Apocalypse 12 Satan is described as waging war on Mary and her children: "the dragon was angry against the woman: and went to make war with the rest of her seed, who keep the commandments of God".
Besides, what you are saying is that one among the last words of Christ on the cross was something temporal and purely economic, without importance to the redemptive work He came to accomplish.