What exactly is '"Moras Ru'ach"?
Rashi (Melachim II 14:26) #1: Pain (i.e. aggravation).
Targum Onkelos: Disobedience.
Rashi #2 and Targum Yonasan: A rebellious spirit (Refer to 26:35:2:1.)
In what way were Esav's wives a Moras Ru'ach?
Rashi and Targum Yonasan: Besides worshipping idols, everything they did was with the specific intention of aggravating Yitzchak and Rivkah.
Ha'amek Davar: They were painful to each in a different way. They knew that Yitzchak loved Esav, so they were not afraid lest he get angry. Also, their idolatry pained him more than it pained Rivkah, who was used to it in her father's house. Esav's wives knew that Rivkah does not esteem Esav, so they showed that they are not concerned for her.
Why did Yitzchak not rebuke Esav?
Seforno: Because he did not recognize the extent of Esav's evil. 1
Which explains why he set out to give him the Berachos, the cause of so much bitterness and hatred.
QUESTIONS ON RASHI
Rashi writes: "[The wives of Esav were a source of frustration] to Yitzchak and Rivkah" (Moras Ru'ach l'Yitzchak ul'Rivkah) - They would worship idols." How is this derived?
Gur Aryeh #1: The verse could have been phrased in the reverse, 'l'Yitzchak ul'Rivkah Moras Ru'ach,' thus equating the reactions of Yitzchak and Rivkah. Rather, it indicates that Yitzchak became upset first [which teaches that the frustration was due to Avodah Zarah]. 1
Gur Aryeh #2: Surely, their frustration was over outright deeds [that the wives committed], and idolatry is what the Canaanite locals were known for.