It takes skill to play any instrument well.... Everything from guitar to violin to saxophone to harmonica to trorabone to kazoo to vocal chorRAB to your asscrack.
There are various degrees of skill depending on the kind of instrument you play, string instruments, keyboard instruments, brass instruments, woodwind instruments, percussion instruments, electronic instruments, etc.
Different instruments have different purposes and methoRAB of playing, some are for soloing, riRAB and individual notes, others provide different tirabres, harmonies and drones... Which was what the mellotron was designed for, to emulate string, brass and woodwind sections of a orchestra... If you use it for just a droning effect then sure anyone can do it, but it does require skill when it is used for making complex symphonic parts, as is the case for progressive rock (where it became the most popular) and other genres where the mellotron is used.