Rowling was always a mediocre to bad writer, it's what makes her work such a gift to the fanfiction writer.
The series is straight and white because Rowling never strays far from Harry's viewpoint, and he is white and straight.
She'scared of her fans, hence her u-turn on Ron/Hermione - because a vocal minority of fans favour Harry/Hermione.
As to Dumbledore -throughout the series he's clearly as camp as a row of pink tents, so how come she had to 'out' him? Because she's the same age as me, but still thinks that being camp is the same as being gay, whereas the younger generation don't draw that inference so they see Albus as just a bit eccentric.
And if she doesn't give us ethnic and sexual diversity, she goes at it with hammer and tongs on the psychology! Harry is bipolar, Ron has ADHD, Hermione OCD, Snape PTSD, Dumbledore is a sociopath, Ginny a psychopath, Luna has Aspergers or something close to it and Voldemort has daddy issues! Oh, that's not right? Should've learned to write characters, then!