My image feel low res

Hi, Sorry to bother you again I still did not found a way to correct this issue. My image feel kind of low res in my game, it is moslty visible when we see line in angle and they are … well, not well define, you can check the picture to see what I am talking about, there are kind of visual artifact who are kidn of anoying. My question is what cause this is it the game resolution? For the moment, i dont adjust the resolution in my game, at the start of the game, the game simply goes full screen.

Any Idea how to fix it? Thanks