Add the code below to Custom CSS field on your Popup widget’s Settings tab:
@media(max-width: 500px){
.global-styles, [class^="WidgetBackground__Container"], [class^="Popup__PopupCloseInsideInner-sc"] {
transform: scale(0.9);
}
.global-styles, [class^="Popup__PopupCloseInsideInner-sc"] {
top:30px;
right: 20px;
}
}