In this case, I think the reveal of the other cat at the end helps make it funny, even if it had been done with a more horizontal aspect ratio. I don't like the "cell phone" look for most photos and videos, though. Why waste the space showing a blurry version of what you are already watching? And, like you said, why can't they turn the camera to get horizontal shots?
I hadn't heard of Milana Vayntrub. Thanks for identifying her! I found a picture I can really relate to:
![actor-milana-vayntrub-sheer_squirrel_girl.jpg](./download/file.php?id=31173)
- actor-milana-vayntrub-sheer_squirrel_girl.jpg (44.14 KiB) Viewed 90 times
I am not sure if it is composed, though, since the heading under the picture was for some sort of fan casting thing.
This pic looks less doctored, and wikipedia confirms she played the role of "squirrel girl":
![scaled_milana_vayntrub_squirrel_girl.jpg](./download/file.php?id=31174)
- scaled_milana_vayntrub_squirrel_girl.jpg (50.23 KiB) Viewed 90 times
While looking at her listings on wikipedia, I noticed that she was in a Key and Peele sketch. It's pretty funny (as most of them are):