So apparently a character (which I dont remember the name) reads Hit's mind and he finds out Hit thinks Jiren is the strongest mortal in all the universes. Hit has seen UI Goku and Hit has fought Goku SSB Kaioken. Also supposedly this character which reads Hit's mind is shown to think the same later (perhaps due to what he readed from Hit's mind).
In the anime we see Jiren fighting Cumber at an apparently same level and Cumber claims "they are even". In the anime Cumber is defeated by Ultra Instinct Goku, though he didnt turn into SSJ3. I believe in the manga he did. Also Gogeta SSJ4 Xeno with much struggle managed to win a beam clash agaisnt Cumber SSJ3.
We also see Jiren being in a similar level with Zamasu, but later we see him overpowering Zamasu.
As far as I remember, candidates to be equally or stronger than Jiren are Cumber, UI Goku, Xeno SSJ4 Gogeta, and perhaps some others such as this guy who is able to bring him to the ground with a gravity related power, Fu perhaps, and may be some others I'm not aware right now.
Is Jiren the strongest mortal character in Dragon Ball Heroes?