It is rare for a pokemon to develop a body which resembles human clothing, and typically when they do, they resemble the clothing of the culture around which they evolved. Hitmonchan's skirt and tunic seem bizarre to most: skirts are women's clothing now, were never worn by boxers, and there are no female Hitmonchan. Put in the proper context, however, it is no odder than a Jynx's robe.
Hitmonchan gained their clothing over two thousand years ago and a vast continent away in the age of Rome. Their ancestors had already become quite humanoid and developed their trademark gloves, but this did not serve them well: their resemblance was not to the "civilized" Romans, but to "barbarians" who wore few clothes and were seen as targets for conquest and enslavement. Many Hitmonchan suffered the same fate as these "barbarians", differing from them only in that they had not formed large armies or used weapons before their capture. Some brave ones fought legions with only a single fist until they were felled in battle.
None of these ancestors have survived. Modern Hitmonchan descend from what was initially a mutation as rare as shiny pokemon. Instead of a bare chest and boxing shorts, some Hitmonchan were born with what seemed to Roman eyes like a cross between a toga and a military uniform, albeit in pink instead of the Roman colors. These Hitmonchan were rewarded for these "efforts" at assimilation (despite being born that way) by becoming among the most popular pokemon among the Roman Senate and a sign of prestige across the empire. When Rome fell, its conquerors adopted many of its ways, among them the training of (now all pink-skirted) Hitmonchan which in time spread as far east as Kanto.