Funny. Jello still slides to the bottom of a container, unless the container is airtight and the jello makes a perfect seal.
As for your trap: Why would someone jump down into that pit and take the damage, instead of lowering a rope and climbing down? Or grabbing their bow and simply killing the Zombie from a safe distance?
Want a variation? Pit in the floor, 10 x 10, with a message scrawled on the wall above it in blood: "Trap...", with the blood trailing down from the last letter, towards the pit.
What are the odds that someone will walk up to the edge and lean over to look inside? Something approaching 100%?
So what would you like to have happen when they do? Should the floor tip under their weight and dump them in? (Just as it did for the guy writing the warning sign...) Should the Zombie who's been waiting at the bottom with a crossbow shoot them with surprise?
In one version there was a Neo-Otyugh in the pit and a giant spider who lived overhead. The spider would drop a net of webbing on the person, the N-O would grab the web and drag the victim into the pit, and the two monsters would split 50/50.
In another version there was a block of stone that would drop on them as soon as their head came over the pit.
As a general rule though, it's important to remember that traps are a lot more fun for the DM than they are for the players. They're the spice in the mix, not the meat and potatoes.