The full saying is "Jack of all trades but master of none is oftentimes better than a master of one"
In DnD it's certainly true, it's called versatility and the funny thing about paladins is they're still a master of burst damage that also comes with being a master of burst healing.
There's only one more busted class and it's called a cleric who is, surprise surprise, also a frontline caster.
Interesting and something I shall ignore anyways as the modern saying is in fact its complete form