The information in the article below contains spoilers for the BFDI series. Please watch the episode(s) if you have not seen them yet.
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 splattered on David's face when the Announcer threw it at him. It was recommended by epiczano.
Apparently, no one likes yellow tomato as they showed disgust to it.