I think it's fair to call out a movie or show for being a bad adaptation
What is bad?
I work in a high end bottle shop where we sell many types of beers, wines, whisk(e)y, what not...
I have people coming in saying 'smoked/peated whisky is disgusting and bad'. No it's not. YOU don't like it. The flavor profile isn't to your liking. This doesn't mean it is badly produced.
Just because YOU don't like a thing, doesn't mean it is bad by itself. Take a step back and don't factor in your own ego. Look at in from afar. Was the production bad? Was the acting bad? Was the cinematography bad? Was the editing bad? Was the writing bad?
Just because you think a character was dull to YOUR personal liking, doesn't automatically mean the whole show/movie was bad.
Look at the movie 'The Room'. This entire thing was badly written, badly acted... the whole nine yards.
But when you look at for example Thor Ragnarok.... This is a movie that goes many ways in terms of humor and how the characters changed over years of getting to know them. Some people didn't like certain choices that were made. This doesn't mean the movie is a bad adaptation. Some people just didn't like it. That is called a weird little called..... PERSONAL PREFERENCE.
And apparently some people still don't understand that what THEY like isn't actually fact. It's opinion.
So, to get back to your statement...... There is hardlly ever something called a 'bad adaptation'. Take Dune. Some people loved it because it hit all the rights notes THEY loved in the novel. Some didn't like it because what THEY liked in the novel wasn't there.
Always remove your own bias when objectively judging a show or movie. But be an adult and be capable of saying something was well made but not to YOUR liking.