While Tarun Matara's duties are unknown, there has been some speculation that the role leads to the sexualization of young girls, though whether that is inherent in the role or simply the result of holding up a young girl as a goddess figure is not clear. Most notably, Amita is completely against making Bhadra or any other girl the Tarun Matara. She accuses Sabal of wanting to marry Bhadra, which he neither confirms nor denies. Ishwari herself was only about 12 years old when Mohan, who was 20, married her, though the marriage was arranged before she became Tarun Matara. A man at the Chal Jama Monastery mentions wanting Bhadra to bear him five sons. King Min's Kyrat entry on Jalendu Temple characterizes the Tarun Matara ceremony as involving "innocent girls [who] were taken from their families and paraded around by leering priests"; given Pagan's relationship with Ishwari, it's possible that description echos her own feelings about it.
Since the Tarun Matara is the spiritual leader of all in Kyrat, Sabal expresses great interest in ensuring that Bhadra knows how important her role is in his post-credits cutscene. The method of which he shows the importance of her role is through the ceremony of purification where he slaughters those who were devoted to Amita, this could insinuate that the Tarun Matara has a clear interest in seeing justice served to those who oppose the leader of Kyrat. This reasoning could also provide a motive to Ishwari's killing of Mohan Ghale when she was the Tarun Matara as he was the major opposition to Pagan Min.
The Tarun Matara's Sleep is location that implies that it's a graveyard for those who have been crowned Tarun Matara.