The Yellow Tomato is the "cake" given to the winner against the 30 recommended characters that got the votes to join Battle for Dream Island in the Cake at Stake for Reveal Novum which-so happens to be David. It exploded on David's face when the Announcer threw it at him.
One of Ruby's sisters is named Diamond, however she doesn't look like this Diamond and has limbs.
Apparently, no one likes Yellow Tomato as they showed disgust to it.
However, Fanny says "I want that Yellow Tomato, I really do.", indicating she likes it.