I am sorry it took a while for an answer to come your way, and I hope it's still relevant...

Domitian was indeed not related to Nero, instead he was the last of the Flavian dynasty, a family who had taken control of the empire after The Year of the Four Emperors. Vespasian, the first Flavian emperor is generally viewed as a benevolent ruler who was loved by most. This generally positive image is however not entirely trustworthy because it was a complete creation by Vespasian himself. As the newest emperor he engaged in a process called predecessor denigration. He tore down the legacies of emperors before him who were deemed unworthy to elevate his own status to legitimize his rule. Vespasian's benevolent image was propagated after his death by his two sons, Titus and Domitian. I tell you this because while Vespasian used predecessor denigration to destroy the legacy of those before him (emperors like Nero) Domitian on the other hand was the victim of this practice some decades later.
Vespasian had brought peace and stability to the empire and was loved for this. He had a strong relationship with the Senate as he allowed them to maintain a sense of their importance and dignity by including them in the decisions of empire. Domitian however was different from his father in that respect. He often went over the heads of the Senate and ignored their opinions entirely. The illusion of power they had had under Vespasian came crumbling down as Domitian ruled the empire without much care for their (self)importance. This created a quite hostile situation towards Domitian and he made many enemies eventually leading to his murder at the hands of his own courtiers.
The Senate, who had been complicit in Domitian's so called reign of terror, saw an oppurtunity to wash their hands of him. They initiated memory sanctions (which we now call damnatio memoriae) and sought to A) expunge Domitian from the history books, B) tarnish his remaining legacy, twisting all positive aspects of his life/reign into something negative. Nerva, the succeeding emperor had an equally valid reason to denigrate the legacy of Domitian. Since he took over after the murde ron Domitian he had to ensure the support of the people and the soldiers by explaining why his reign would be better than Domitian's. The best way to do that was by pointing out how horrible Domitian had been. Nerva and the senatorial elite engaged in a serious campaign of memory sanctions against Domitian and tarnished his legacy to the best of their abilities. I will spare you all the ways in which Domitian's image was destroyed by a hostile Senate and hostile succeeding imperial dynasty, but one of the methods used was by framing Domitian as a terrible and malicious tyrant. Nero had already suffered a similar fate after his demise and comparisons between an already known tyrant like Nero and one they wished to implicate in similar behavior, Domitian, served to cement this tyranical image.

So in short: Domitian and Nero were compared with each other because both were the victims of a process called predecessor denigration, which was used to elevate one's own position to legitimize their rule. Nero was framed as a tyrant, and by comparing Domitian to Nero, he too was framed as a tyrant.