Honestly, I wish this mentality would disappear.
People are naturally conservative and there is some innate instinct that respects stubbornness. That sticking to your guns is, on some level, admirable even when all the evidence is against you. But we have plenty of innate instincts that we should ignore because we know they are problematic, and this is one of them.
People who choose a side of an issue and then stick to it are lucky if they are later vindicated, but that's usually not likely. People often make the wrong decision using rationales that are basically inescapable. I grew up in a homophobic environment. How was I supposed to realize that there was nothing wrong with being gay when the entire narrative was that it was, and you were punished for even questioning it. Hating gay people was just a thing you did, and since I was straight and all my friends were straight as far as I knew, I really had no reason to question it. I could have figured it out a long time before I eventually did using pure reason, but no one exists in some kind of existential bubble of logic and reason and we're all affected by the culture and situations we live in. And I'm lucky I developed the skills to question these values and think independently. Some people, sad to say, don't do that, and I'd say that's more of the reason we are so divisive than anything.
The fact is that as people live their lives, it's really better to be able to adapt than not. Sometimes you get lucky and you're on the right side of history when you look back on things in hindsight, but most of the time, people need to change their views in order to become better people. Hillary Clinton being against gay marriage at one point isn't nearly as strong a point against her as her changing her view to be for gay marriage is a point for her.
So, no, it doesn't speak to her poor judgement. It's a sign of good judgement to be able to question and adapt your beliefs into something better.
You could argue that perhaps she's only changing them to appeal to the public, but then you still have a president that is going to advocate progressive values. If she is a shill, then she's a shill for the right side. But the truth is we can't know whats inside her head and there's no reason to assume the worst. I'd be willing to bet that she holds the progressive principles genuinely, because she atleast holds them for years at a time, and when you pretend to hold those positions for that long, then it's not really shilling, it's a genuine change. And that should be applaudable.