By default, in Divi, the Add to Cart icon does not appear on the mobile menu. If you want to add the icon you could add a snippet to the Divi Theme Options Custom CSS area.

Go to your WordPress Dashboard > Divi > Theme Options > Custom CSS and add the following:

@media (max-width: 980px) {
#et-secondary-menu {
display: block !important;
}
}@media (max-width: 767px) {
#et-secondary-menu .et_duplicate_social_icons {
float: left;
}
#top-header .et-cart-info {
float: left;
}
#et-secondary-menu {
width: 145px;
margin: 0 auto;
}
}