If you compared Harriet to Becky Sharp who is beautiful, poorly born but clever as anything, the prospects would change.
There are many times where beauty captivated despite birth and rank and poverty. I think that was the case with the Bennets and even Marianne. Harriet is just so silly and ignorant she wouldn't be able to make a good marriage on her own. I like the Harriet from E4 because she looks most like what I imagine from the book.