Overall, I think your design aesthetic is working pretty well at the moment. It's relating to the concept of the Office pretty well with a consistent aesthetic and your imagery of the characters supporting the quotes they say. I would say the legibility of the text on mobile is the only thing I would watch out for. Especially for the name, the font size gets pretty small on the mobile viewing screen, so I would definitely try and bump up the readability on that. Additionally, on the 3rd postcard, the leading on the quote feels too open so definitely either tighten it up or change the font size to make it one line. Another thing I would possibly consider, is to not use red type on the first postcard. Currently it's a bit distracting since its fairly different from all the black type you use. I know you're using it because of the paper sheet background, so if possible, maybe try and find another image or a another way of handling the contrast of text to the background?
Currently, the design's visual priorities are given to the characters from the Office from just the focus on their faces to the relatively small type in comparison. Whether or not you want this to be the case is up to you but it is worth noting. The only design element that I would watch out for would probably be the spacing of the text in relation to the image. On the 2nd postcard, the text moves pretty far across the screen as the scale changes, and you start to lose the relationship between the two. No transparency is currently I think, but it is worth looking into to help with legibility of the text on background.
For the technical side of things, I would watch the font sizes as the scale changes. Right now, the text is getting marginally bigger from the desktop to the phone screen, but I feel that you should emphasize the shift a bit more to both preserve readability and to give more weight to the quotes because they're honestly really good and funny. I would watch how the text moves though as you scale it. I would be wary of how it clips with the imagery and how far its moving across the viewing screen. Especially on the 2nd postcard, the legibility of the text on the background in the phone layout gets a bit messy with the blur in the background so maybe try a transparency scrim. Also, for image edges, they mostly look good, just watch Jim's ear on the 2nd postcard. Other than that, I think it looks good and that you just have some small things to tweak so it shouldn't be too bad!