Hi,
With the popups it's not easy to manage for mobile devices.
What can be done is to edit the css properties of the iframe in your template css file in order to make it responsive.
The HikaShop css is not loaded in popups.
Another way could be to disable the terms and conditions default field. Create a new order custom field with the type "checkbox" and set only one value like "MY_TERMS_AND_CONDITIONS", set it as required and finally add a translation in the override part like:
MY_TERMS_AND_CONDITIONS="<a href='
thelink.to/your/article
' target='_blank'>Please accept the Terms and Conditions before proceeding</a>"
This way it will open the terms and conditions in a new window / tab instead of having a popup.