Stretch part of image at angle?

[PixLab]

(08-22-2023, 08:20 PM)c4cc Wrote: https://ibb.co/c8NrwMN I can't increase layer size

YES YOU CAN, just look further or move the window to the left

   

(08-22-2023, 08:20 PM)c4cc Wrote: Sadly, the resolution is somewhat low? Or do I combine the high pass layer with the Sharpen (unsharp mask )layer?

I'll quote rich2005 as it seems you did not read its post
"Ah..such is life. Any sort of bitmap image translation - scale - rotate - skew - etc can degrade an image."

Let's try to sharpen that anime, go to Filters > G'MIC_Qt, a window opens, in the top search field input "sharpen", and try one of those 3 in the G'MIC plugin, default setting should be OK

   

If you don't have G'MIC plugin in GIMP, get it there > https://gmic.eu