It's hard to form a clear judgment of Mary and the Giant in the context of either the 1980s, when it was published, or today. It's a book of and about the 50s that couldn't have been published in its own time. There are a complex combination of things going on here: intentional racism by the antagonists, unintentional racism sneaking in because of the time, an appalling indifference to Mary's sexual abuse at the hands of her father (even as an adult), all wrapped up in the recurring theme of rescue that could either be plot symmetry or the 50s idea that the solution to every girl's problem is being carried somewhere by a man. Seriously. It's like they think Mary can't walk, despite having seen her do so many times.
But. The story itself, of a small-town girl trying to escape herself before ultimately learning that where you go, there you are, is quite interesting. Like Dick's sci-fi, it's fast paced and totally readable, and many of the characters are relateable and enjoyable. I'd have liked a little more backstory on some of them, and a more detailed epilogue, but Dick was never a man for details. That forthright directness is what keeps Mary and the Giant from descending into standard romantic drivel. And the lack of drivel is, in a nutshell, what kept it from being published for 25 years.