There's the explicit gag itself, and it's an old one, of stepping on a scale. The real joke, however, is in fact that it is the president. It's more humorous

There's the explicit gag itself, and it's an old one, of stepping on a scale. The real joke, however, is in fact that it is the president. It's more humorous because it shows the president as a real person. It's unexpected, and out of context of how we typically understand the holder of the office of the president of the united states should behave. That's the main joke. So yes, the finer details are important. It's not just a funny picture.