If I am understanding correctly, when the media is on the right, on mobile the image is below the text, but you want the image to be above the text.
This CSS, added to Customize > Additional CSS will put the image on top:
@media (max-width: 600px) {
.wp-block-media-text.is-stacked-on-mobile.has-media-on-the-right .wp-block-media-text__media {
-ms-grid-row: 1;
grid-row: 1;
}
.wp-block-media-text.is-stacked-on-mobile.has-media-on-the-right .wp-block-media-text__content {
-ms-grid-row: 2;
grid-row: 2;
}
}