... The reason why opening principles are important is because active and centralized pieces lead to tactics, and lack of activity prevents tactics. So when your position is far more active, you should be looking for tactical opportunities. If you are completely stuck, there are three basic places to go:
1: Look for checkmate threats that cannot be stopped.
2: Look to attack pieces your opponent can't defend
3: Look to fork pieces that your opponent can't defend both of4: Look for pawn breaks to open the position. This will favor your more active pieces. …
... dumb question: If I dont understand what an opponent is doing in the opening phase (which is most of the time), should I just keep following the opening principles? Or do I change course and take my best guess on how to counter?
There is no simple answer. For one thing, it depends on the quality of your guesses. Chess is something of a balance between principles and reaction to what your opponent is doing. Ignoring either one is risky. After 1 e4 e5 2 Nf3 Nc6 3 d4, the usual move is 3...exd4, reacting to 3 d4. After 1 e4 e5 2 Nf3 Nc6 3 Bc4 Nf6, the move, 4 Ng5, seems like a violation of principles as usually stated, but a lot of serious (and also a good many not-so-serious) games have been won that way. As you gain experience, you are likely to improve at getting the balance right (and being able to understand what your opponent is doing). It can help to look at illustrative games in books.
There are a lot of resources out there. What's mainly important is that you are exposed to as much good chess as possible!
... what do you mean by "good chess"? Does this mean that playing against similarly rated opponents who blunder as much as or more than me is "bad" chess that I should avoid? ...
I do not know what EOGuel meant, perhaps he was thinking of looking at illustrative games such as those that one might find in books like:
Simple Attacking Plans by Fred Wilson (2012)
https://web.archive.org/web/20140708090402/http://www.chesscafe.com/text/review874.pdf
http://dev.jeremysilman.com/shop/pc/Simple-Attacking-Plans-77p3731.htm
Logical Chess: Move by Move by Irving Chernev (1957)
https://web.archive.org/web/20140708104437/http://www.chesscafe.com/text/logichess.pdf
The Most Instructive Games of Chess Ever Played by Irving Chernev (1965)
https://chessbookreviews.wordpress.com/tag/most-instructive-games-of-chess-ever-played/
As for one's own games, common advice is to seek opponents who are a little better than you, but I don't think that very many would expect you to rigidly avoid "similarly rated opponents". I would think that most would recognize some value in practice at avoiding blunders and exploiting the blunders of others.