Most Fanfiction is sort of like a "What If", they are about how they spin the characters.
The reason why Dumbledore is constantly turned evil or a manipulator in fanfiction is... because in the books he does almost nothing BUT constantly manipulate everyone, albeit for more altruistic reasons, and rarely steps in to help even in life or death situations in spite always knowing what is happening.
The competent writers with few exceptions (which they themselves will often explicitly note) don't
Heck the last Harry Potter Fanfiction I read actually has a lot more interesting, vibrant, and down to earth, Hermione (or rather I like their Hermione more then the books)... and the POINT of that Fanfiction was that everyone's personality was derailed. It starts off with everyone being cardboard cut outs, in fact a few are (Since the fanfiction was about all the Harry Potter characters being fanfiction facsimiles of themselves who were slowly forced to become more 2-dimensional and then 3-dimensional).
The sign of incompetent writers to me isn't whether or not they use cardboard cut outs. It tends to be whether or not the fiction sort of goes out of its way to play out its theme. Does it, for example, make a character younger JUST so they are within the age range of our lead? Does it make one character evil because they were antagonistic towards the character focused in the fanfiction?