Was Mahalath a Canaanite and did Esau marry her to get back at his father, Isaac?
"When Esau was forty years old, he took as wives Judith the daughter of Beeri the Hittite, and Basemath the daughter of Elon the Hittite. And they were a grief of mind to Isaac and Rebekah" (Genesis 26:34-35).
Esau married two Hittite women, people who did not follow God. The Hittites are descendants of Heth, the son of Canaan. So Esau's first two wives were Canaanites. They were a constant irritation to his parents. So when it was time for Jacob to marry, Rebekah insisted that Jacob go back to her own people to find a wife.
"And Rebekah said to Isaac, "I am weary of my life because of the daughters of Heth; if Jacob takes a wife of the daughters of Heth, like these who are the daughters of the land, what good will my life be to me?"" (Genesis 27:46).
That this took Jacob way from Esau and his pledge to murder Jacob after Isaac died was an extra bonus to Rebekah. Isaac agreed with Rebekah and sent Jacob away.
"Then Isaac called Jacob and blessed him, and charged him, and said to him: "You shall not take a wife from the daughters of Canaan"" (Genesis 28:1).
Esau watched all of this. He still thought that he could somehow win a favorable blessing from Isaac.
"Esau saw that Isaac had blessed Jacob and sent him away to Padan Aram to take himself a wife from there, and that as he blessed him he gave him a charge, saying, "You shall not take a wife from the daughters of Canaan," and that Jacob had obeyed his father and his mother and had gone to Padan Aram. Also Esau saw that the daughters of Canaan did not please his father Isaac. So Esau went to Ishmael and took Mahalath the daughter of Ishmael, Abraham's son, the sister of Nebajoth, to be his wife in addition to the wives he had" (Genesis 28:9).
He thought he could fix things by adding yet a third wife, this time marrying a cousin (after all Jacob was being sent to marry a cousin). However, we see that he didn't bother to travel far and didn't really consider the character of the family he was marrying into. Like many people, Esau thought he could fix a problem that he created by doing more of the same thing.