You can achive this by custom css. In Interface option please provide the class name and with that class you can change the property of Popover content.
A popover in ionic (at least the Angular API) likes to receive the $event in order to know where the user clicked. It will the center around this info, as by default a popover is something that should pop close to a user click event location. If not present (that event data), it should present in center.
But not sure if that is happening here. But it could explain some of the magic happening, which also hinders using CSS to change this behavior
And this deals around the Popover API, not the Select_show_as_popover thing