Is it poor sportsmanship to resign immediately after a major blunder early in the game? In a recent (online) game in which I had black, my opponent played an opening variation with which I was unfamiliar. That, a very aggressive attack, and lack of adequate time to think through my moves had me down a pawn and in poor position as we entered the middle game. While I wasn't yet in major time trouble, they had significantly more time on their clock also.

After a relatively long time before their 15th move, I got the end of game sound and pop up window. The first thing I noticed was that my opponent had taken another pawn and put me in check with their queen. For just an instant, I thought I was being told that I had lost. However, I had multiple ways out of check so knew that couldn't be correct. Then I realized that my opponent had resigned so fast after making their move that the move and end of game signal appeared on my computer simultaneously. Apparently they hadn't seen that I had a knight guarding the square to which they had moved their queen.

Presumably my opponent resigned because they had hung their queen and realized it right after making the move. While that mistake devastated their chances of winning the game, to me it felt unsporting for them to have resigned so abruptly, when many pieces were still on the board (in addition to the two pawns I had lost, the only pieces off the board were from a bishop/knight trade).

Is a resignation like this bad sportsmanship? Or should I just accept it as part of the game after such a significant mistake? (For reference, both of us are rated in the 1100's.)
It is totally normal, and actually, I would consider it good sportsmanship, but that is a bit relative to your level of play.

Among masters, if someone blunders even a piece, they know that their master opponent will have no problem converting, so to play on could be considered rude and a waste of time. That said, they have the right to, and if they do, you don't really complain about it.

At MUCH lower levels, sometimes players are not strong enough to win, even up a queen, so playing on may make sense.

In either case, it is your opponent's right to resign a position they do not want to play. Take the rating points, and smile.