Is Imam Ibn Kathir a Hadith scholar or a Qur'anic exegete, and is he a mudallis? And if he is not a mudallis, then what is the authenticity of the Hadith about Tha'labah, of whom it was said that he died in Uhud?

Ibn Kathir was an exegete and a Hadith scholar. No one among the specialists in the field, neither Ibn Hajar nor Al-Suyuti, mentioned that he was a mudallis (one who practices tadlis, i.e., conceals defects in a Hadith chain). Moreover, scholars from the eighth century CE onwards are not described as practicing tadlis. It is feared that the questioner might have confused him with Yahya ibn Abi Kathir, who was a mudallis. As for the story of Tha'labah, it is not clear what the questioner finds problematic in it.