HD 1080 is the resolution of modern standard television. There’s will be no pixelation in HD. Of course you can zoom in to see the pixels in Animation Paper, but that is on purpose so you can check the details of the actual pixels. This is perfectly normal.
If you need to output in higher resolution – if you want to do 4K for cinema viewing or monitors that can actually display that, then you can go to 4K. But the point is, you don’t need to work in 4K. You can work in say, 2K (or even 1K), and then scale up to 4K as the last thing you do before exporting. Drawings and lines done in Animation Paper will scale up perfectly without degradation or blur.
Working in 4K is demanding unless you have super hardware. No need to. So just work in 2K and export in 4K. Much better to have the fast feedback.