어울리다 does three related jobs here. It is fitting in with people — 사람들 사이에서 쉽게 어울리지 못했어요, 'she couldn't easily fit in among people'. It is two things harmonising — 자연과 도시가 어떻게 어울릴 수 있을까, and flavours balancing. And it is something suiting someone: aespa가 My Name을 해도 되게 어울릴 것 같아, 'it would really suit them'.