How do I turn off auto-aim in Genshin Impact?

Is there any way to turn off auto-aim? Or at least have a way to lock on to enemies?

No, and no.

Free targeting is not currently an option in the game for auto-attacks and most Elemental Skills/Bursts. However, some skills feature ground targeting, which you can activate by long-pressing E, such as Fischl's and Amber's Elemental Skills.

Also, if you are a Bow user, you have the option to activate Aimed Shot by pressing R, which allows you to control the crosshair and move it exactly where you want to target.

Other than that, targeting seems completely random.