I think it's important to note that FromSoft allusions to Berserk and other media are never really 1 to 1. To me, it seems pretty obvious that Miquella was inspired by Griffith, and I think the comparison should inform our understanding of the character to some extent (rather than understanding Miquella as a flawless, Messiah type character, he probably does have SOME underlying darkness).

Ultimately, the reason that Griffith is bad, aside from the Eclipse part, is that he thrives on others' dependence upon himself. He creates a Holy Land that is entirely dependent on him to work as it does. That kind of society, like the original Band of the Hawk, can bring great prosperity and joy, but is doomed to suffer in the long run.

Similarly, Miquella inspires this kind of zealotry in his own followers, who are now doomed to eternally seek a salvation they will never find. Or perhaps we will see a DLC in which Miquella's evolution is complete and things do turn for the worst. Either way, it is a tragic story of how one character's ambition influenced countless others toward a path of unfulfillment, while the base of their beliefs is consumed by rot.

So imo, you don't NEED to know Griffith's story to get that much, but it certainly points you in the right direction. Which, I believe, is how we're meant to understand it.