The way our play group interpreted this was to calculate caster level as per PHB pg. 164 (I.E. add levels of Wizard, Sorcerer, Bard, Druid, 1/2 levels paladin or ranger, 1/3 Arcane Trickster, Eldritch Knight). The character spell casting level is the level used to determine cantrip effectiveness.
E.G. Monk 14/Warlock 3 has a weak Eldritch blast (level 3). Whereas Sorcerer 14/Warlock 3 hits like a truck with an Eldritch blast (level 17).
High elf class feature cantrip was hotly debated, but follows the same rules as above.
Basically if you are a magic user you are good at magic, if you are a level 17 High Elf Fighter you won't be incinerating foes with fire bolts.