Depends.
If you quit after you lose 1 city, but you still can build / get reinforcements, than quitting is very ennoying, just because your chance to win is diminished.
If you quit after a lost attack you shouldnt quit, same as above. Plus your opponent wants revenge.
If it is obvious for both parties that playing on no use has, quitting is ok.
If you can only camp in 1 city, it would be even apprecieated by the attacker if you quit, because ai is stupid. However, it is fun to hold on as long as you can.