To fix your font-weight issue, here's the code:
.mktoForm .mktoRequiredField label.mktoLabel{
font-weight: normal;
}
As far as responsive goes, Forms 2.0 are responsive, but the CSS code you have above declares fixed widths for the form and the fields. If you remove that code, your form should be responsive. Here's a screenshot of some tweaking I did in Chrome Inspector:
Desktop: http://cl.ly/image/1M3h1R2a2J0r
Tablet: http://cl.ly/image/1g3P42082r13
Mobile: http://cl.ly/image/032f33113b3u
You just need the coders to understand Marketo a little more 🙂