It's true that Sinkees are too serious and can't take jokes and sarcasm. That's after decades of PAP brainwashing and propaganda which make them unable to see Singapore from the perspective of an outsider.
The Americans can openly joke about their president, senators, congressman, and their own country and it's no big deal to them because it's a free country with their right to free speech. Some of even burn the American flag in public and it's OK in a free society.
Imagine if someone burns the Singaporean flag in Orchard Road, I bet 9 out of 10 Sinkees who see the incident will immediately call the police to arrest the culprit. A police state also requires the full cooperation of a brainwashed citizenry to rule over them.