Jell-O is generally not halal because it usually contains gelatin derived from animal sources, often from pigs or animals not slaughtered according to Islamic guidelines. Gelatin from such sources is considered haram. However, there are halal-certified Jell-O products available that use gelatin derived from halal sources, such as bovine or fish, or gelatin alternatives like agar-agar. To ensure the product is halal, look for a halal certification symbol on the packaging, which indicates that the product has been vetted by a reputable Islamic authority.
And Allah knows best.

Sources:

• IslamQA – Is Gelatin Halal
• Sahabah – Is Jello Halal
• Halal Home Cooking – Is Jello Halal or Haram