Scenario:
- Open the list which has a lookup field.
- Click on 'Customize forms' command under 'Power Apps' in the command bar at the top.
Yes, it is possible.
You can customize the SharePoint list which has lookup columns in it.
Here are the examples of others using lookup columns while customizing list forms:
Update:
If you are not able to customize the list forms using "Customize forms" option in command bar, follow below workaround: