Spouse & I have talked about it, & that would be our definition of "success" for this creative project: separate creative works that are made with artistry & care that share a feeling of continuity.
It's fair enough that you should watch the original film before a direct sequel (e.g., probably you will want to watch Superman before Man of Tomorrow when it comes out), but I would be fucking mad if I felt like Supergirl relied on people having seen Superman. I think that's cheap & lazy.