Is it possible to get into a Top 20 school with a poor part of your application?

Yes. It’s what the entire thing looks like when it’s all taken together. But if one of those things is low, the other parts have to be even more compelling.

And - I suspect that while you can get in with a less than near-perfect stats (GPA, test scores) as long as the depth/type of ECs, LORs and essays are outstanding, the opposite is not generally true. IOW, I don’t think many of the 4.0/1600/36 kids who have weak ECs or a bad LoR or meh essays are getting in.