What is the Fatwa about a person who intentionaly says to his wife that she is his mother?
Is it Zihar or not?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If a husband means by the words to his wife 'you are my mother' to make her unlawful to him just like the person who makes his wife unlawful to him by Zihar (saying one's wife that she is like his mother's back i.e. unlawful to approach for enjoyment) then, he is obliged to pay the expiation of Zihar.
Allah says: {……in that case (is) the freeing of a slave before they touch each other. That is an admonition to you (so that you may not return to such an ill thing)…..} [58: 3]
If he means by such words that she is as respected, honorable, and beloved as his mother and does not mean Zihar then it is not considered as Zihar.
However, one should avoid using such confusing words. Some scholars believe that every one who says to his wife: my sister, or my mother should be refused and punished.
Allah knows best.