This is a great question. Let me start off with a little plot to show you how Hyllus is a tragic figure. Heracles falls in love with Iole and he even besieges a city to obtain her. When Deianeria find out about her husband's adulterous escapade, she is angry and she wants to take matters into her own hands.
At this point she remembers that Nessus, a centaur, gave her a potion before he was killed by Heracles. Nessus stated that the potion, which was mingled with his blood and the poison of the hydra would make Heracles only love her. So, she dips a robe in this potion and sends it to Heracles, in order to win him back.
When Heracles puts on the robe, he suffers immensely (and eventually dies). Deianeria also realizes that she has been tricked by Nessus. The potion was a fatal poison. When Hyllus hears about this, he upbraids his mother. She commits suicide.
After his Hyllus finds out the truth that Deianeira did not intend to kill her husband. So, he is filled with remorse at his words. In addition, He tells his father the story and Heracles realizes the truth as well. Finally, Heracles makes Hyllus marry Iole.
In light of the summary, we can see that Hyllus is tragic in the sense that he aided in the suicide of his mother, when she was innocent.
He is not tragic in a sense, as well, because Hyllus did not possess a noble characteristic that led to his demise, as typically happens in Greek tragedy.