Good guy. You probably would have forgotten for much longer and gotten a realticket in about a month. You want real idiots in uniform, try the Maryland State Police.

See, I go back and forth on this, but mostly I'm thinking jerk for having stopped you in the first place, because it's entirely likely that while he had you pulled over to get your little warning, three people whizzed by with tags or stickers that are MONTHS overdue or with cars that are registered in the wrong state because they think they're pulling a fast one or something. I don't go a single drive without a "where's a cop when you need one" incident - the answer is that they're protecting us from one day overdue stickers.

Was he a good guy for so doing or was he a jerk for having stopped me in the first place?

Depends. Was he playing Good Cop or Bad Cop? If he was polite and respectful and never seemed to seriously consider ticketing you, then I'd say he was a good guy just doing his job. After all, a one-day-overdue sticker usually means "I forgot," not "I don't bother." So, he reminded you, as inoffensively as a guy with a gun and a badge possibly can. OTOH, if he was brusk or otherwise unpleasant and made you feel like "be grateful I'm not throwin' the book at ya, punk," then I'd say he was being a jerk.