OK. Thank you. You use Kobo Touch right? 600x800 screen if I remember correctly.
Can you please try this two files and share your opinion about quality?
This one is resized to 600x800 so quality of zoom should be very low.
http://pawelj.vulturis.eu/Shared/Ubunchu!-NoZoom.cbz
This one is 900x1200. So 150% zoom should look perfectly. I have no idea if you can set exactly 150% zoom on Kobo.
http://pawelj.vulturis.eu/Shared/Ubunchu!-150Zoom.cbz