I'm not sure why this is really a question. The answer should be: Both so that players have options to tell the story they want to tell.
In truth, however, 'mundane' is never really mundane. If you look at the epitome of the 'normal person' characters in comics you'll see them exceeding the limits of reality often. Whether it is Godlike accuracy of Hawkeye, the insane athletic capability of Batman that far exceeds the limits of Olympic athletes, the inexplicable ability of Black Widow to get through alarm systems and locked doors as if by magic, or every hero's ability to recover from life altering wounds overnight - without scars.
In my setting I have an explanation for why a Champion or Battlemaster is better than a town guard. PCs, and some important NPCs, are Godtouched. The Raven Queen murdered a Dwarven Death Deity and wiped his name from existence. When she did so, she ascended to Godhood - and immediately gave up the power by shattering it and placing it into tens of thousands of mortals. This allowed them to 'advance' as PCs do. Without the Godtouch, it takes a person 70 years of work to master a single third level spell. Elven non-Godtouched mages might master a 5th level spell over 700 years of effort. However, the Godtouched can rise from nothing to mastery of 9th level spells in the span of a year or two.
When it comes to fighters, barbarians, monks, rogues and other potentially 'mundane' classes, this explanation allows me to treat them more like Captain America - someone that is theoreticlaly mundane, but is so good at what they do that they seem magical. I tend to really play up that these PCs are doing the impossible when they achieve many of their class abilitiy limits.