When I add my form to my Wix website via the HTML app - using the iFrame code, on mobile view why doesn't the form doesn't optimise itself for mobile?

So you are trying to make your wix form optimized for mobile on your wix website and it is not working?

Usually what happens in this case-type is that wix is not reading your form to format it appropriately because it is embedded within a page that is already intended to be optimized on wix. However, with your form being embedded unless the page itself is the container to show your mobile type form to fit with a certain specification then the optimization will get ignored.

There are 2 ways you can fix this problem. Add a the Mobile Responsive Widget to your Form. Or alternatively, you can just set your Form Theme to Mobile as a standard across-the-board styling. Both of which should help to resolve this problem that you are having.