Mixing ability can differentiate a good DJ from a great DJ. However, first you need to be a good DJ. A good DJ knows how to curate and select. A good DJ consistently selects great tracks that work well together. A good DJ can read the energy of a room and match that with their selection. A good DJ can use the tracks to compose, to tell a story. Simple, and even botched transitions can still sound good if the tracks being mixed work well together. However, mixing ability cannot fix bad selection. But when you add great mixing atop great composition skills, that is when a DJ set can become a cohesive experience. And only then will you understand how to mix in a way that detract from the music.