Is there any difference between "I didn't find it funny in the slightest. " and "I didn't find it funny at all. "?

No, they both mean that it wasn't funny. They are absolute statements, they don't allow for any amount of funny.

[@corta] 🤔not especially they sed aclear message that you did not find what they amusing at all and need too take that on board you here not pleased

No, it's the same thing.

The first one is used when making comical reference to Elon Musk and the second one is used when mimicking the speech pattern of Donald J. Trump.

No