Hi there,
A short answer would be, yes.
When you make an Ad Request, the request would include all the information, which includes the device's current orientation, that would enable you to serve the best possible ad at the time of the request. So if you sent an Ad Request while the device is currently in Landscape, it will return an ad that would also be in landscape mode. The same case would apply when the device is in Portrait at the time of the Ad Request.
That said, if you have already forced your application to always be in Landscape mode, then all your Ad Requests should return Landscape ads.
Regards,
Joshua Lagonera
Mobile Ads SDK Team