You are mixing two separate things here.
1. The image on the retina is flipped and our brain flips it back. That’s just how lens works: if you focus light on a surface, the image gets flipped, light coming from the left passes through the pupil and gets focused on the right side of the retina and vice versa (including up and down). All non-faceted eyes will work like that, mammal or cephalopod.
2. Our retina is inverted with photosensitive elements on the back side and nerves on the front. Yes, that’s suboptimal. But evolution doesn’t always find an optimal solution first try and a suboptimal one will often get too developed to switch it for anything else. Cephalopods were lucky, we got stuck with suboptimal eyes. There’s a lot of such things in living creatures and they are considered one of the proofs of evolution because intelligent design of species would imply a very bad or drunk engineer.
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