[eftpotrm]

OK, there's two separate things here :-)

Trumpet fingering is easiest in C, F or G majors (as read, concert Bb, Eb and F). The actual blowing is quite physical and needs a good bit of practice to build strength; that is easiest in C, and the lowest fifth of that too.

But.... There's only three keys, they always operate in the same sequence, so once you've learnt that fingerings flow quite easily, and the harmonics are good, useful intervals. Perfect fifth, perfect fourth, major third, minor third. That gives the five core open harmonics over a core range of two octaves once you use the valves. Hence, once you've got yourself going a bit, while some keys are easier than others the instrument's structure lends itself nicely to switching keys at will without major issues. It makes it a nice instrument for improvisation.