So the Site you imported added some CSS in the Additional CSS which is overriding some of the page hero settings. Go to Customizer > Additional CSS and look for this Rule at the very bottom of code:
.page-hero {
padding-top: calc(15% + 130px);
}
Remove that, then you can adjust the header elemts top padding for mobile and on the Header Element > Site Header tab under merged header you can reduce the offset.
Just above that piece of CSS you will see this:
@media (max-width: 768px) {
#page {
margin: -75px 15px 0 15px;
}
}
The -75px is what pushes the page content up and over the header element. Make this smaller to decrease the overlap.