Actually,
This is a perfect example of how resignation can be taken as either respectful or rude. This all matters who you're playing with in my opinion. Some players might think it's an insult, some might not. I wouldn't worry about insulting anyone from a resignation. If you feel the need to resign, resign.
No it's not rude. But it may not help you improve depending on the size of the advantage, rating and time controls. At beginner ratings and fast time controls, victory often goes to the penultimate blunderer. An opponents advantage might evaporate in a few moves. At slower time controls and intermediate ratings, being down a whole minor piece isnt always decisive. Maximise activity, threats, counterplay, and critically evaluate further trades. Consider sacrifices to open up the king - what have you got to lose. Hanging on in tough positions can teach you a lot about chess and builds resilience.
Thanks for your replies. I usually would want to carry on to see if I can even things out as a learning experience. I'm only playing 15|10 and daily as I find blitz bewildering and hopeless.
Resigning is not rude. In fact, many consider not resigning in a 100% losing position as rude.
If you blunder your queen and/or losing horribly, it is considered good sportsmanship to resign.
I think it is wrong for a beginner player to quit just because of a material advantage.
It's time to resign when your opponent is going to slog through a bunch of unstoppable pawn promotions. The time to resign is when it is hopeless AND it is going to take a long time. That would be the time that not resigning is rude.
Otherwise, you are robbing yourself as a beginner not to play to mate. Sure your opponent has a material advantage, but how is your opponent going to use their material advantage? Do you know? If they used their material advantage badly, how could you have done it differently? How did they mate you? Could they have done it better? Losing is winning, because it makes you stronger, so you should lose as much as you can for as long as you can.
Heck, they may even blunder and even out your material advantage. 
Actually,
This is a perfect example of how resignation can be taken as either respectful or rude. This all matters who you're playing with in my opinion. Some players might think it's an insult, some might not. I wouldn't worry about insulting anyone from a resignation. If you feel the need to resign, resign.
But no, youngster, the Chess World does exist already, and has it's own etiquette and manners no one can just show up and claim "to each their own". You could all the same try to tell us "thank you" is polite or offending depending on how we see it. Quit spreading false information, thank you.
To resign is only weird if it's not obvious one has a big advantage; but it's never offending or rude by itself.
To play on a totally hopeless position without even the hope to flag your opponent, will damage your "gentleman's" reputation. But that's all. (Other players will then whisper nasty names in your back, such as "But what an idiot!" etc)
I feel like it’s also a sign of respect for the other player.
As a previous commenter said, things seem a bit different for a beginner. I think it is rarely a foregone conclusion at this level. I have a propensity for blundering my queen but have won quite a few matches in spite of that. Its good to learn all situations, from both sides. How does somebody learn to finish somebody off without practice?
On a related note, is it frowned upon to play for a stalemate?
no, instead, you try to flag them. Even if they're up by 15 points.
No, resigning is normal and is respectful. There is no point in playing a position that offers no hope.
no no it's much better to resign when losing material because the other player doesn't have to put in effort to avoid your counterplay
It's not rude either way. Every grandmaster reference I've seen though encourages you to keep playing. You will learn more from playing and having to find resources in a position than by resigning. Also, you'll be surprised at how many of those games you can win/draw.
For titled or expert players, it is pretty much expected that a player in a no win situation resign, especially in long time controls. For intermediate and below, maybe not so much. I have drawn losing games and seen folks ahead on material blunder it away especially when they get careless and over confident.
New to chess.
If my opponent has taken an early advantage e.g. in material, is it considered rude to resign and avoid a long drawn-out probably foregone conclusion?
Thanks