Asleep was the way Harry liked the Dursleys best; it wasn't as though they were ever any help to him awake.
I have a bit hard time to understand the second part of the sentence, especially "it wasn't as though". Can it be paraphrased as: "it was as though they were not ever any help to him awake" ? What does it mean exactly?
You will find "It wasn't as though ..." is a relatively common idiomatic expression to mean
It wasn't true that ...
or
It never happened that ...
often contrary to expectation. Examples:
It was not as though he was the fastest runner, but over longer distances the horse could maintain his pace remarkably well, and often finished well ahead of the other horses in the race.
i.e. you would think the horse should be fast to win races, but this was not true.
"It was not as though I could move all these desks on my own," she thought to herself. "You'd think the headmaster would send some students to help out."
i.e. despite the headmaster's expectation that she could move the desks, this was not true.
Rowling's sentence basically means, "contrary to what you would expect from legal guardians, the Dursleys were not of any help to Harry when they were awake" -- a colorful way to say that they were of no help to Harry at any time, but they were least unhelpful when they were sleeping.
The complement of as though expresses a non-actuality, a non-fact, just as the complement of as if does.
With the big holiday sale, it is as though an army of zombies was pushing at the shop door.
As a form of irrealis, it can be marked by a backshifted verb, as with was in the zombies example above.
When negated, BE not as though, it's often employed in contexts where someone is weighing the pros and cons of a situation and some convincing is required, that the "downside" is imaginary, not a real problem.
C'mon, man up! Just open the door and let them in. They're only bargain hunters. It's not as though they are an army of zombies.
I know it's something of an inconvenience, but if you can pick me up at 4pm sharp, I can be waiting right on the corner for you. It's not as though you would have to search all around for a parking spot and then come find me.
In your example, as the narrator is revealing Harry's thoughts, we see Harry in the act of considering the pros and cons of the better state for the Dursleys to be in, awake or asleep. Deciding in favor of "asleep" is not a mistake since...
It wasn't as though they were ever any help to him awake.
