Nah, that's a deliberate writing technique. Standard mystery writing; "who's the girl who Harry is going to hook up with"--can't make her too obvious, or else people will guess. So...you downplay the target for a while.
But what if "people", by which I mean people buying your books would prefer an obvious character that's unique and likeable to a mystery character that's unspectacular in most regards? I mean maybe it didn't matter to her since it was her final book and the money was already in her bank (brilliant!!), but I know I wasn't the only one a little miffed at how these things went down.
So...if you don't like such writing techniques, you absolutely should blame Rowling.
Fine, but blaming Rowling for an unspectacular character lacking substantial development whether intentional or otherwise doesn't make said character any more spectacular or developed. In fact it makes me more miffed that measures weren't taken to make her a more engaging, more likeable character. As such, she is near Ron-tier for me which is not a good place to be.
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Arcanine: The best pokemon ever.
Dewott: The cutest pokemon ever.