Monk dips are bad unless you have high WIS or CHR and usually they're still bad.
They are often not necessary. But calling them bad? How can ~20 extra AC and likely a boost to saves be bad? You don't sacrifice BAB here and even get a bonus feat from this deal. And can even flurry with certain weapons, at no AB loss.
Monk dips are almost never bad. Broken - maybe.
Again, you can't evaluate it without looking at the opportunity cost or diminishing returns. As I said if you've got the high WIS/CHR that's a different conversation but most people monk-dipping don't. You still have to want a bunch of extra AC which you don't need if you're playing tight and you don't get when you need it (flatfooted, since you haven't attacked yet so haven't turned Fighting Defensively on). And what saves are we talking about? Can still critically miss with 100 Will. Ounce of prevention's worth a pound of cure.
General principle is monk-dipping is for soloing/unfairing ch1 without scaling, neither of which is really playing the game as designed. You pay the price for that early defense by retarding the progress of your class for the rest of the game. Offense has nearly infinitely increasing returns (like accuracy in Deadfire), defense doesn't.