A good April Fool’s joke is one where the person you’re playing it on bursts out laughing when they figure out the joke. Where at no point do they feel scared, uncomfortable, hurt, or betrayed.
A good April Fool’s joke won’t trigger anyone’s epilepsy, PTSD, or anxiety; or make them question if they’re having a hallucination, delusion, or dissociative episode – and you can’t assume that you know for sure that those things aren’t an issue for someone.
Good April fools jokes are also ones that don’t require a lot of effort on the part of the person being “pranked”.
Randomly putting stickers of rubber ducks (or whatever inside joke/thing you personally find funny) around the house? Okay, that’s great. Shoving their purse/drawer/etc full of confetti or filling a bathtub up with water balloons? Not so much, unless you know the person won’t mind. Don’t make messes for other people to clean.