What is the ruling on holding the Mushaf and reading from it behind the Imam during Tarawih prayer, and does that diminish the reward?

It is permissible to follow the Imam from the Mushaf (Qur'an) during Tarawih prayers and other prayers. There is no evidence indicating that this is disliked (makrooh) or prohibited. Many jurists have permitted this, such as Abu Yusuf and Muhammad (though they considered it disliked), and Shafi'i, Malik, and Ahmad (who considered it not disliked). They based their rulings on the actions of Anas ibn Malik, whose servant used to hold the Mushaf for him, and the action of Dhakwan, who led prayer for Aisha. Furthermore, holding the Mushaf is no more than the Prophet, peace be upon him, carrying Umamah during prayer. However, one should be careful not to read aloud with the Imam. In summary, it is permissible, though avoiding it is better to steer clear of scholarly disagreements.