All my customers that are imported into QBO show up as tax-exempt, how do I make it so they are not tax exempt?

I want to make sure you'll able to fix the tax status of your imported customer, Mark.

If sales tax hasn't been set up in your QBO, imported customers will default to tax exempt.
Moreover, in import mapping, there isn't a tick box for tax exempt so we have to edit the customer profile one by one and change it to non-tax exempt.

Here's how: 

1. Go to Sales.
2. Select Customers. 
3. From there, locate the customer and click the name to open.
4. Then, click Edit.
5. Scroll down then under Additional info, uncheck This customer is tax exempt.
6. Repeat the steps to all imported customers.
7. Once done, click Save.

 In addition, I've included this article to provide more information on how you can modify or restore deleted items in QuickBooks: Edit, delete, and restore list elements in QuickBooks Online.

If you need further assistance with any QuickBooks features and processes, please don't hesitate to drop a comment below by clicking the Reply button. We are dedicated to help you 24/7. Have a good one.