Is it permissible to fondle one's wife by massaging her feet in front of her mother, as a gesture of appreciation, given that she does not expose her awrah (private parts)?

Good companionship is required between spouses to increase intimacy and affection, as Allah Almighty says: "And live with them honorably." This includes kind words and good deeds, as the Prophet (peace be upon him) said: "The best of you is the best to his family, and I am the best among you to my family."

What the questioner mentioned about joking and playfully interacting is not forbidden by Sharia. However, doing so in front of the wife's mother depends on custom, and whether it is normally considered a cause for shyness or not. The general principle is to avoid anything that offends modesty and is customarily a cause for shyness, so spouses should not do such things in front of people.