I can't speak as to whether it was always frowned upon, but commentators on etiquette during the Renaissance did name a lot of nasal breaches of good manners, including picking, blowing, and wiping. This was true both in Northern Europe and Italy.
Erasmus's De Civilitate Morum Puerilium (1530) was a handbook on manners directed at children. (Check out this beautiful edition from 1544). He has a specific section devoted just to the nose:
Drying or wiping your nose with your cap or clothing, or on your sleeve or arm, is a habit only for fishmongers. Not that it's much better to wipe your nose with your bare hand if you're only going to wipe that on your clothes. Rather, it's good manners to remove any dirt from your nose with your handkerchief, and, if more distinguished people are present, to do it with your head turned away. Source. BONUS: The relevant passage in the 1544 edition, below the TEXTVS heading
Meanwhile, in Italy, Giovanni Della Casa wrote another treatise on manners called Il Galateo (1558). Here's what he has to say:
And when you have blown your nose you should not open your handkerchief and look inside, as if pearls or rubies might have descended from your brain. This is a disgusting habit which is not apt to make anyone love you, but rather, if someone loved you already, he is likely to stop there and then. Source
Edit: Messed up the links