Mercer still exhibits a whole bunch more “fallibility” across the board than most of the ST captains across the franchise history. The man just did a “flyby” to check on his ex!
And the “bumbling”? What bumbling? Again, the pilot has been consistently described as one of the best in the fleet. Others had similar accolades to their credit or unique attributes, including one of the few Xelayans serving on a Union ship and the only Kaylon.
They weren’t “bunglers”, just written with their foibles on more prominent display. The verbal jousts, in-jokes and asides we see between several of the characters typically existed only within a single tight pair or trio of friends in a given Trek series- Kirk/Bones/Spock, Bashir/O’Brien, etc.- if at all.
Besides, we’ve gotten to focus on the supporting cast a lot this season: multiple Moclan storylines, a necropsy-holoromance, goo-guy done good, family-centric stories for 2 different families, a spy revealed and (seemingly) redeemed, and a cast change.