I suppose you're to be forgiven for not knowing the extent to which Trump's "thousands of thousands of celebrating Muslims" lie was researched, but it's worth noting that dozens of research organizations -- including several conservative ones -- tried to track details down. Insofar as there are "sides" here, both "sides" have a vested interest in actually discovering the truth of this claim, and no one has been able to corroborate any celebrations consisting of more than a handful of small, isolated groups (of around five to fifteen people), several of which turned out to be nothing more nefarious than someone's birthday party. (There were a couple cops in Dearborn who claimed to have been called to investigate a large rooftop celebration at a local mosque, only to turn away when they found the door on the ground floor locked, but since there's no police report or even a dispatch record to back that claim up and one of the cops involved has a couple, um, problematic tattoos, I'm comfortable with leaving that in the "hearsay" column.)
Now, if Trump had said, "I was listening to talk radio," he might have been telling the truth, as a handful conservative radio shows did falsely claim to be broadcasting audio of Muslim celebrations of the towers falling. But that wasn't his claim; he claimed to have seen thousands and thousands of Muslims celebrating in the streets of Jersey City, and no such footage ever aired.
Is he crazy? Is he senile? Was he misinformed by his handlers? Why do you believe he chose to lie about this?