'Weekend warrior' in slang dictionaries
The earliest slang dictionary notice of "weekend warrior" that I've found is from Robert Chapman, New Dictionary of American Slang (1986), which treats it as interchangeable with "Sunday soldier":
Sunday soldier or weekend warrior n phr A military reservist or member of the National Guard, who typically goes on uniformed duty at the weekend
The same entry, garnished with an origin date of "(1950s+)," appears in the most recent (fourth) edition of Dictionary of American Slang (2007), which is a bit surprising, given that the English speakers have clearly extended usage of the term to apply to people who pursue various nonmilitary activities on an occasional or part-time basis.
Jonathon Green, Chambers Slang Dictionary (2008) captures this broader rage of meanings:
weekend warrior n. 1 {1960s+} (US) members of the National Guard. 2 {1970s+} anyone deemed to be insufficiently dedicated to a given activity or occupation. 3 {1980s} (Aus.) a member of the Australian Army's Reservist units. 4 {1980s} (US drugs) one who takes potentially addictive narcotic drugs on weekends (or similarly special occasions) only 5 see WEEKEND HO above ["{1970s+} (US black) ... 2 (also weekend warrior) an underage prostitute"].
U.S. slang dictionaries were rather slow to pick up on the existence of "weekend warrior" as a slang term for an military reservist. Indeed, the earliest "weekend X" term to appear in the Dictionary of American Slang series was "weekend hippie," in the second supplemented edition (1975):
weekend hippie = plastic hippie ["A part-time, or "week-end" hippie who likes to adopt on occasion hippie attire and mannerisms, but who is not committed to the hippie life-style or beliefs."]
From Green's definitions 2 and 4 and from the 1975 Dictionary of American Slang's definition of "weekend hippie," it appears that the modifier "weekend" (like the modifier "Sunday") often appears as a marker for someone who is not whole-heartedly committed to an activity, occupation, or lifestyle. Nevertheless, a weekend commitment isn't the same as no commitment—and it is certainly possible for people who don't devote any time to an activity, occupation, or lifestyle to acknowledge this in at least a somewhat respectful sense in their use of "weekend warrior."
The early days of 'weekend warrior'
As DavePhD notes in a comment beneath the posted question, circumstantial evidence suggests that "weekend warrior" may have originated in connection with U.S. Naval Air Reserve units. From "Bloomington Man on Annual Summer Trip," in the San Bernardino [California] Sun (August 6, 1950):
A member of the Naval Air Reserve's "Weekend Warrior" Fighter Squadron 776, C. B. Bell, aviation machinist's mate, of 636 Taylor street, Bloomington, is now on his annual summer cruise.
During this year's cruise, Bell's squadron is operating under simulated combat conditions, including gunnery, bombing and rocket firing, night flying and all other training phases.
And from "Reserve Pilot Killed in Crash in Berkeley Hills," in the Santa Cruz [California] Sentinel (January 22, 1951):
Berkeley, Jan. 22 (AP) — A graduate student of the University of California was killed yesterday afternoon when his naval reserve fighter plane crashed and burned on Grizzly Peak in the foothills just above the university campus here.
Dead is Lieut. V. J. MacNeilage, of Berkeley, a "weekend warrior" of the navy's organized reserve fighter squadron 873 at Oakland naval air station.
He was on a routine training flight when his F6F "Hellcat" fighter smashed into the 1759-foot peak in Tilden regional park on Berkeley's eastern boundary. A low overcast limited visibility at the time of the accident.
Whether the term "weekend warrior" was originally a derisive appellation attached to naval reservists by full-time U.S. Navy personnel or an admiring appellation attached to them by full-time civilians is impossible to tell from these instances; but it is clear from the way in which the term appears in quotation marks and initial caps in the "Bloomington Man" story that the reservists themselves proudly took semi-official ownership of it. Moreover, it would have been in incredibly poor taste for the "Reserve Pilot Killed" story to have alluded to the man who died as a "weekend warrior" if that term were widely viewed as a put-down in the early 1950s.
My sense is that "weekend warrior" was viewed as an honorable, if ironic, term for a reservist at this period—five or six years after the end of World War II and a year or two before the Korean War became a major conflict. As for today—when the term has evolved to refer to someone who seriously pursues some athletic or other physical activity away from work or school—the tenor of the expression can be admiring or self-effacing, or it can be belittling, depending on the speaker and the situation at hand. I have heard it used both ways.