Is winning on time bad manners?

The clock is part of the game.

You should resign quickly if you are going downhill on the board. This makes the game fun for your opponent. 

Do not take notice to such comments. Time is an essential part of the game. If you think you can beat your opponent on time, then by all means do so. If your opponent can't handle the outcome because he/she was maybe a queen ahead, that is his/her problem, not yours. So no, it is not bad manners. Frustrating for your opponent? Yes. But again, that is his/her problem.

Time is part of the game.

I'm an old guy who plays 10 minute games.  If I'm up a queen or a rook , or if my opponent has been lost for ten moves , and my opponent nevertheless manages to avoid checkmate for two minutes or so and then wins on time , I consider that to be unsportsmanlike and cheap.  On the other hand , if my position is lost , or even if I'm down a pawn without compensation , I'll resign like a gentleman and live to play again.

Nothing unsportsmanlike or cheap about it. Time is a part of the game. And you can use it to your advantage. And as for your own approach to what seems like a lost position;

It is a matter of "gamemanship" vs "sportsmanship," and both types of players are on a chess server.  It's just plain wrong for either group to deride the other.  And, if you play blitz with any sort of delay, then a player should be able to win with a huge advantage.  If you can't, then get better.  As for myself, I'd rather lose a game where I had played sharply and correctly, but then got swindled in my time trouble.  And, if I've played like a moron in the early part of the game, I get little solace by winning via some time-induced "howler" by my opponent.  But, those are the two extremes, and most games fall in between.

At your level, you never resign.

And its up to you to win the game.  If you cant, that's not your opponents fault.  It just means you need to work on your game.

It is not a matter of anything. Time is a part of the game, period. Why do you think chess is played with a clock?

If you're talking about chess at classical, or even rapid time controls, then I agree;  But blitz chess has that element of "chimp chess" -- make a move, get a banana.

The same goes for blitz chess

No one has ever won a game by resigning
then your opponent is caught cheating
(not saying anyone here is)

So technically it should be "no-one ever won a game by resigning on the board."

And... Lasker said, "When you see a good move, look for a better one."  Lasker never played bullet.

The clock is a weapon as much as your pieces are.

No. imo it's stupid and childish to keep playing a ridiculously lost game in the hope of flagging your opponent by making silly moves but it's going to happen.

I do think the "YOU MUST NEVER RESIGN!" stuff that people parrot on here is misleading. Fight on, sure, I've hung my queen and managed a victory by launching a completely unsound attack, and had it done many times to me, I can only tip my hat if you manage that. Pre-moving your king all over the board isn't playing chess though, it's also kind of evident that people that win this way are the most likely, in my experience, to start trolling you immediately after. That's the type of play that's being encouraged with the "never resign!" stuff.

If your opponent is mad abt losing on time they shouldn't be playing five minute blitz