Whenever i use a texture pack in a world,then overide it with another texture pack,it made a world more pixelated,uninstall and reinstall the game only made it worse and it happended to me twice in the day
Heres how to reproduce it:
First,use any resource packs(in my case,faithful 64x)
Secondly;add it to a singleplayer world
Thirdly, add a texture pack above it
Fourthly,get in the game and see the difference
Fifthly,uninstall and reinstall the game and immediatly create a new world upon opening the game,and sees it change for the worse
The image below is real and i have un/reinstall the game twice