If this is using a HubSpot theme, then the form will take the width of the container it is set in when the page loads. So you would need to ensure that the container on your website has a full width on mobile to display your full form.
If you have set as a raw html form, the form will inherit all it's styles from your website's styles, so these styles on mobile would be set on your wordpress stylesheet and will need to be updated there.
Tom Mahon Technical Consultant | Solutions Engineer | Community Champion Baskey Digitial