[Vuejs]-When Vue Component mounted hook called, have all the DOMs inside already been rendered?

Look at the official Vue documentation for its lifecycle.

According to this, once you reach the mounted hook the DOM will be fully rendered and done. It’s the very last hook you can attach to that isn’t an event or an exiting function.

So in short: Yes, once you reach mounted everything is ready and good to go, the rest is just related to updating or exiting Vue.