It depends! Rhinos vary a lot in size and weight across species. A Sumatran rhino can weigh as "little" as 700 kg, an Indian rhino as much as 3,200 kg. It also depends on the specific Olympic-size pool you're talking about because they have a minimum depth of 2 m, but 3 m is recommended for multi-discipline use. The density of jello depends on the amount of sugar and gelatine in it, but Wolfram Alpha lists it as 1,141 grams per litre.

Thirty Sumatran rhinos would weigh 30*700 kg = 21,000 kg, and thirty Indian ones 30*3,200 kg = 96,000 kg. At a jello density of 1.141 kg per litre, that's 21,000 kg / 1.141 kg/l = ~18,405 litres of jello (~84,137 for the big ones). At their minimum depth of 2 m, 50 m length, and 25 m width, Olympic-sized pools hold 2,500,000 litres of water. 18,405 l / 2,500,000 l = 0.007362, or about 0.74% (0.03365, or 3.37%, for the big rhinos).

So, 30 grown rhinos are as heavy as 0.74% to 3.37% of an Olympic-sized swimming pool filled with jello.

This photo was taken in the state of Victoria in Australia. I checked out a few of trams in Melbourne, and one of their lightest models is the Z-class, which weighs 19,000 kg, and one of the heaviest is the E-class at 62,000 kg. The phrasing "as much as" and the lack of a specific rhino species makes this poster a little ambiguous because there's a wide range of possible weights, but technically, they aren't lying.