Donk betting in No Limit Hold 'Em
How you should be thinking about donk bets when deciding whether to use them:

Donk bets don't have an inherently set purpose.  You can do it as a bluff, you can donk bet for value, or you can do it to try to influence behavior (like a block bet).  What you're trying to obtain by doing it depends on your hand and your expected response from your opponent.

As a thinking player, you shouldn't ever just check solely because the other player was the preflop aggressor.  Like any other decision you're faced with in poker, you should only be choosing to check if you believe that you will get more value from the hand by checking than from any of your alternatives.  Whether you believe this depends on your opponent.  For example, if you're holding a hand which is likely ahead, rationally choosing to check requires something like believing that your opponent will bluff continuation bet more often than they will call your donk bet.

Specific extreme examples: If you were playing against someone who does not bluff continuation bet on the flop, but had a strong tendency to call with weak hands, then you should donk bet every time you hit a decent hand on the flop, and rarely bluff them.  If you were playing against a person who rarely calls unless they have the nuts, yet bluffs all of the time, then you should check to them every time when you have a strong hand, and bluff donk bet them every time you don't.

Not only are you sometimes missing value if you decide never to use donk bets; such a decision would be an exploitable aspect of your play.  It gives a larger advantage than is due to the preflop aggressor, as they can be confident that they know what at least one of your moves will be in a following betting round (you will check).  In particular, they can derive more value from raising preflop cards which are geared more toward drawing (e.g.: suited connectors like 7s 6s), because they know that they will usually be able to check behind he flop if they missed, so they actually get to see 4 board cards before being faced with any possible aggression.

Why is it perceived as bad:

Inexperienced players tend to make bad donk bets (bad independently of being donk bets), and commit extra money to the pot in situations where they're generally already at a disadvantage by being out of position, or where board cards suggest that their hand is not as strong relative to the preflop aggressors as they may think.

Also, some people perceive donk bets as sort of against poker etiquette.  They think that "check to the raiser" is the proper, gentlemanly thing to do.  This is more of a cultural thing, and has little to do with trying to play profitably.

They are not inherently bad, and there are situations in which a donk bet is the optimal choice.

How players misuse them:

Players misuse donk bets the same way they misuse any other sort of bet.  They make their decision without considering likely hands their opponent is holding or their opponents tendencies.