When mirroring a component or sub-assembly in an assembly you have to remember that SolidWorks mirrors relative to the center of mass of the component or sub-assembly. If you are not aware of this you can get some unexpected results.
I have created an unsymmetrical component and inserted it into an assembly to demonstrate.
Now we will use the mirror tool.
What you will notice is that you will have several different orientations you can choose from plus a “Create opposite hand version”. More often than not you will probably just want to use the opposite hand option.
If you would like to learn more about this you can look at S-049636 in the SolidWorks Customer Portal, https://customerportal.solidworks.com/