Spinning off a Frumhouse discussion on the topic, hair was a powerful idea for men and women in the Torah.
On the one hand the Sotah’s hair is loosened and unbound. On the other part of the Nazir’s restraint involves growing out his hair and not cutting it. For the ultimate Nazir, Shimshon’s hair is literally the seat of his power. Paradoxically the Kohanim shaved their hair before entering service in the Mishkan. As does the Nazir at the end of his Nezirut.
Hair on the head was a symbol of power, an extension of the body. And power could be restrained or unbound. The Torah has a number of prohibitions on shaving and destroying hair. This represents the tension between the extremes, excessive asceticism and excessive hedonism. The Nazir was one extreme and the Kohen was another. Most people fall somewhere in between.
Hair on the body tends not to be referenced. Unlike Greeks, Jews were not overly concerned with personal grooming except in the Hellenized upper class. Sanhedrin 21a does feature the quoted view of R. Yitzchak but the context of the Gemara is clearly not hostile to Tamar. The larger picture is that David Hamelech’s own indulgences created the complicated family situation that resulted in rebellions and Tamar’s rape and the fight for power.
David Hamelech himself has been compared to Esav who was called the hairy one.