According to the Mahabharata, after entering the Svarga (heaven) with a human body on Indra's invitation, Yudhishthira witnessed that Duryodhana "was seated on a beautiful throne and he shone with the splendour of the sun and around him stood in attendance the goddess of heroism and other entities of righteousness". Yudhishthira found this insufferable and reminded the dwellers of Svarga about his sinful deeds.
Following that, Narada smiled at Yudhishthira and explained that Duryodhana had suffered for his sins, and that ultimately, Duryodhana was a warrior who had defended his dharma and fought bravely and valiantly, having been a great ruler, a true friend, and a terrible foe.
Was the happiness of Kauravas in heaven real or an illusion?
Narada words indicates that the happiness of Kauravas was real, i.e: they have some sins but also they have good deeds.