You just need to capture the approval status via your workflow, save it in a variable and showing it on Nintex form. I might need to ask you more information about the scenario you are trying to achieve. Are you talking about approval status column in SharePoint? Or you are looking for task outcome?
I want to achieve this functionality on a content type form associated with a library.
> We have a content type say 'Custom Content Type'
> We have desgined form for this content type
> This 'Custom Content Type' form is associated with a document library, 'My Library'
> We have set the Content Type approval required for this document library
> Now what we need to achieve is, when the user uploads the document and when he updates the metadata of the document using this Nintex form, the approval status needs to be shown in the form
You can add a calculated control on your form and select approval status from item properties. Please note this is not a column you can edit the value via Nintex form.