You have the software; now you need the content. Since the PSP store is gone, you must source your digital files manually.
The PSP Digital Comics app is picky . It does not read standard CBR or CBZ files. It reads (Sony Digital Comic) files. Digital Comics Psp 6.61 Download-
Bookr (a homebrew PDF and image reader) or PSPComic v1.0 (reads CBZ/ZIP natively). You have the software; now you need the content