The site displays just fine on mobile as is.
1. It’s bloated. In gallery view, I don’t need to see anything but a searchbar and the grid of images. In image view, I don;t need anything but the image, maybe with fave/upvote/downvote buttons showing up on tap
2. Viewing an image in full requires you to scroll down and tap the image
3. You cannot swipe between images, you need to use the arrows
4. If you decided you want to see the image in full, not the top half of it, to even use said arrows you need to scroll up again
5. Each time you go to the next image, the entire site has to reload, there’s no prefetching of the images, nothing of the sorts
The mobile site is usable, just barely, but hardly ideal.
Animeboxes has Derpibooru support, but just for browsing. Can’t fave, rate, read or add comments, etc. That’s the most that can be done with the current sorry state of the API.