My first thought is that that kind of argument is exactly the kind that even a highly rational person would be wise to respond to with epistemic learned helplessness. It involves putting infinities into a calculation about real-world decisions, and it's an argument that all ethical actions are meaningless.

Also, if one values amount of good accomplished instead of amount of good in the universe, the infinite universe doesn't change anything.

That said, I don't think the multiverse interpretation implies an infinite number of universes unless the universe is also infinite in space or time, so people discussing the multiverse may not believe in an infinite universe.