Yeah, the transparent PLA I have is more crystal than transparent. It lets light shine through, so handy for encasing a status LED, but at any reasonable thickness it’s too cloudy / obscured to make out details through.
But I have seen success printing truly transparent panels and things using PETG / PCTG.
Depends on the type of plastic. PLA is what I have and it’s the most commonly used material, and it will definitely degrade in UV light.
ASA is the most common, seriously UV resistant plastic, but it is inherently opaque (and printing it releases styrene which is toxic and requires special ventilation).
PETG is relatively easy to print, can be printed even more transparently than PLA, and has some UV reistance, so would probably be your best bet.
It will still eventually yellow or structurally degrade over time from sunlight though. I also suspect some of that phone case yellowing might not be UV exposure but absorbing oil from your skin.