The extent to which even critical people believe that Kirk proper was at least brash is evident in Strange Horizons's own "Nimoy and Spock: Reflections and Farewells." I cite this example not to drag anyone, but rather to point out the pervasiveness of this conception among people whose critical faculties and interest in the text I take more seriously than I do those of Thewurstboyfriend.
Serious, logical, balanced - [Spock] was the perfect counterpoint to the rash, bold Captain Kirk. Whatever gave you the idea that Kirk was rash?
There is no other way to put this: essentially everything about Popular Consciousness Kirk is bullshit. Kirk, as received through mass culture memory and reflected in its productive imaginary (and subsequent franchise output, including the reboot movies), has little or no basis in Shatner's performance and the television show as aired. Macho, brash Kirk is a mass hallucination.
Erin Horáková mounts a strong defence of James Tiberius Kirk's qualities as an effective starship captain during the Five Year Mission and beyond.
It would probably be more effective at half the length, but it needed to be said, lest everyone assume that the J.J. Abrams / Chris Pine take on the character is the new default for stories about Star Fleet's greatest starship captain.