Worf begins TNG with very short hair, and progressively grows it (see images below). But for someone who thinks of himself to be very Klingon, having short hair doesn't seem to be a Klingon thing to do (again, see images below). Why, then, did Worf have such short hair to begin with? (All I can think of is a Starfleet hair/grooming policy, although that didn't stop Worf having long hair later on, albeit well groomed)
They were actually still developing the character at that point. They ultimately decided that a more long haired look cut a more 'feral' and wild appearance. Also, the bulbous head with short hair looked stupid so they subtly transitioned from the short-to-long hair style by quietly slipping in a pony-tail.
I don't have the source handy, but one of the novels explained it as Worf trying to fit in. Although proud of his Klingon heritage, Worf was still somewhat anxious about what people thought of him. The events we witness throughout TNG helped him to grow more comfortable with himself, and his decreasing anxiety is symbolized by his hair getting progressively longer throughout the first few seasons.
He wore his hair shoulder-length throughout his Academy years, but was often ostracized due to being the first Klingon in Starfleet. He served nearly three years as an Ensign on the USS Aldrin before his transfer to the Enterprise. Presumably he started cutting his hair short during that period.
As others have commented, production-wise it was because they didn't quite have the character's appearance nailed down yet in the first season. The prosthetic for his cranial ridges was updated in the second season, and the longer hair helped hide the edges of the latex application.
In-universe: Starfleet, much like current military organizations, undoubtedly has regulations governing crew member appearance and hygiene maintainance, and requires its members to maintain good grooming, including keeping hair short (probably to avoid letting it get caught in machinery and such). For comparison: Why does the Army require soldiers to have crew-cuts?
Also, Worf was raised by human parents, and many human cultures routinely trim hair (or at least male hair).
Worf's hair isn't that much shorter than Commander Kruge's. I suspect in-universe a Klingon would just assume Worf is nobility (which Worf is).
Out universe we should remember that the Klingons of Star Trek III and IV were the ONLY ones prior to Worf with the modern makeup. TOS Klingons had very short hair and motion picture Klingons also had medium length hair but it isn't quite consistent with what became the makeup of the classic movies to Berman era (though not so far off one couldn't assume ethnic variation).
