A few posts ago I present the image of the gag cartoon of a burglar from Jumbo Comics #1. Kirby scholar Stan Taylor had suggested that it may have been done by Jack Kirby. The cartoon was from page called Jest Laffs. Jest Laffs also appears in Jumbo Comics #2 and two of the cartoons there look like they were done by the same artist as the burglar from JC #1. There are a number of features that are shared, some more important then others. The use of darker regions with a raggy edge or the way the mouth is often placed off to the side of the face. I find the manner of depicting the nose and ears to be particularly interesting. It is in minor details like that individual artists often provide distinct mannerisms.
While the Jest Laffs page in JC #1 provides no credits, the title in JC #2 gives a Bob Kane attribution. There are other gags in the Jest Laffs page in both issues that are done in other styles. This could mean they were actually done by other artists. Or it could mean that Bob Kane adapted his style to one appropriate for the particular gag. After all Kane’s Peter Pupp, also in Jumbo Comics, was very done in a different style than his Batman.
I do not know enough about Bob Kane’s work to say whether any of it shows the same distinctive ears and noses found in the gag cartoons. It does not show up in Peter Pupp but that could just be due to the different nature between Peter Pupp and Jest Laffs. I have also examined much of Jack Kirby’s early cartoon work and could not find those distinctive ears and noses in any of it; including the one Jack did of a burglar.
For me this does not provide a definitive answer to the question of who did these particular gag cartoons but it does mean the Bob Kane should be considered along with Jack Kirby. Jack Kirby only appeared in the first three issues of Jumbo Comics. Although I have seen later issues, I was not examining them in relation to this question. If there are issues later then JC #3 with Jest Laffs gags that share the same traits then I doubt that Jack Kirby would have been the artist, if they stop with JC #3 then that would be another piece of evidence that they were done by Jack.