Before the embeddable template were available, you could purchase a custom built template to make the DIY form look like your website. Those still using a custom skin may find that the form behaves correctly on mobile devices, but your custom template is not.
The custom templates are not built to be responsive. The eCommerce department does not perform edits on these skins so your organization has two options:
Do Not republish any DIY form using a custom built template
Stop using a custom template and use an embeddable layout instead