Get Views field value in Twig template
field.content is the rendered field. And in views that means it is no longer a render array, but the finally rendered markup. So this is very problematic to use it as a class name, not only because of twig debug. 

Better use the row data, where you find the entity object with the field data from the database. Use clean_class to escape it for using it as a class name:

{{ row._entity.field_myfield.value|clean_class }}