Hi,
Are you talking about this plugin ?
extensions.joomla.org/extension/jamp/
Using that extension to fix the width of address input fields on mobile is definitely not a solution.
Normally, you should be able to fix this with a bit of CSS.
JAmp generate static pages. So this won't work properly with ecommerce solutions which need to display dynamic elements on the pages (like the cart, the checkout, etc). So it's indeed probably that extension causing the problem.
If you have an issue with the width of address input fields on mobile, then please provide a link to the shop so that we can check on the situation and tell you what to do.