You could do this w two cross filters on an account report type. But, you couldn't see the contact data without creating a custom accounts with contacts report type.
Standard Report Type
Create a report on accounts (record type equals accounts).
Cross filter one. Accounts with contacts, where important equals false.
Cross filter two. Accounts without contacts, where important equals true.
Cross filters are always AND so this will show you accounts with at least one contact where important equals dalse and without any contacts where important equals true.
Custom Report Type
If you need to see the contact fields, you need to build a custom report type, with primary object = Accounts and secondary object = Contacts.
Then you can create a report, add cross filters as above, and see the contact data. I used email opt out as the checkbox in my testing, but same idea applies.