It doesn't matter if the damage is next to null. Zamasu can attack consistently for ALL ETERNITY without stopping. Also, in due time he will become the entire multiverse and be EVERYWHERE. No matter how strong Jiren is, he needs to sleep, eat and rest eventually. He can't tank or guard the attacks forever and he won't have a safe place for long.
Exactly. Plus don't forget that he wasn't just expanding his influence across the Future Multiverse, he was also invading other timelines. For example, he reached the Present timeline by using the same time rift that Future Trunks and co. had used with their Time Machine.
I actually can't decide whether Infinite Zamasu is more OP in the anime or in the manga. Because he was pretty strong in the manga too. I mean, he was an endless army of thousands upon thousands of Fused Zamases
with the same power level, and they could constantly regenerate and multiply after every hit. Not even Mastered Ultra Instinct Goku could do anything against such an opponent, because it doesn't matter how powerful you are, you cannot destroy that which is immortal and endless in numbers.
But, sure, I am supposed to believe that Jiren could easily annihilate an immortal being who merged with the very Universe, or an immortal being who could multiply into thousands upon thousands of clones possessing the same amazing strength as the original. Oh, and both incarnations of Infinite Zamasu are all immortal, which means no need for food/rest, no exhaustion, no risk of dying, etc.