Yes Jello does hydrate you because when you make Jello it's around 99 percent water.
That water then hydrates you when you eat Jello.
Also Jello turns into a liquid when you eat it as it literally melts in your mouth because your body temperature in your mouth is warm enough to turn the Jello back into Liquid.
Jello is a nice hydrating treat to eat when it's hot out as well.
But you still should drink plenty of water too so that you stay hydrated.
Jello will eventually melt back to liquid form when left out of the fridge for too long.
When you remove the Jello from the fridge and eat it the Jello then melts in your mouth because your body temperature is warm enough to actually melt the Jello and turn it into Liquid.
So if the room temperature gets warm enough and the Jello Sits out long enough as in an hour or two hours or so then the Jello can and usually will turn back into liquid.
Then you would have to place the Jello back into the refrigerator for an hour or so to cool it back off and turn it back into the solid form.
But every time you eat the Jello it always turns back into liquid.