Is Jiren the strongest mortal character in Dragon Ball Heroes?
Hit hasn't witnessed the might of Mastered Ultra Instinct Goku. So it would make sense he would think Jiren was the strongest mortal at that point in time. Taking that into consideration, is he the strongest mortal? Maybe. However, it seems very likely that Goku should be as strong or possibly stronger than him in his Completed Ultra Instinct Form, especially since there has been the indication that he's able to tap into Ultra Instinct. As for the others you mentioned, Jiren didn't seem fazed by anyone he fought apart from Hearts when he lowered the gravity around him. So Jiren, for now, should be one among the top 3 strongest mortals.