That's a fair question.Maybe. The concept sounds good, but lots of things sound good in concept. The issue is the precise mechanism employed:
I don't know how you would break down evidence by percentages like this. Usually, when people say something like "I'm 90% sure he's guilty", there's nothing actually backing that number up, it's just a figure of speech.
How would you decide that "most" of the evidence doesn't serve to disprove self defense, what is evidence? A single witness? A single statement by that witness? What if there's a video, and then a slowed down or enhanced version of that video, is that one piece of evidence or two, or is it more than that?
I'd say that the exact mechanism does need work, but the idea is sound, and could be applied to a larger selection of cases, besides just self-defense cases.
If that question, or something like it, was added to jury instructions in general, that might cut down on malicious/political prosecutions.