I can’t recall ever coming across “having a thing about” being a positive. It’s always negative, as far as my internal sense of the language goes.
“I have a thing for” is positive, but I wouldn’t use it for jeans that you would wear yourself. If you say “I have a thing for skinny jeans”, I’d expect that you like to see attractive other people in skinny jeans, not that you like to wear them yourself.
If that connotation is widely used, it’s not one that I’ve ever paid any attention to. In my experience “have a thing” just means “feel strongly about” and that can be either positive or negative.
I tend to assume positive unless the context tells me otherwise. I think that’s because my first associations for “have a thing” meant some kind of romantic feelings. It wasn’t quite a relationship - maybe just a crush or a hookup (in the days before there was a word for hookups). So a guy might have said “I have a thing for Jennifer” and he probably just meant that he wanted to ask her out, not that they had a definable relationship. It was a while before I remember the phrasing being applied to anything other than people/relationships.
I agree that “have a thing” means “feel strongly about.” Or maybe even more like “I have complicated feelings about.”
It can be more than just like or dislike. You could say “ugh, I have a thing about skinny jeans. I mean I think they make my ass look amazing and I would wear them every day except I can barely breathe when I have them on and I can’t stand the fabric!”
There’s more to that than just loving or hating.
Another example would be “I have a thing about Justin Bieber. I think he’s such an awful person and I hate his hair but boy do I love the way he sings!”
Other people who do not have a “thing” about jeans or Bieber would say maybe they love them, they hate them, or have absolutely no opinion either way.
when you “have a thing” about something you have a strong opinion.
That opinion could be either way.
Some people have a thing about promptness. They dislike anyone being late (or in principle early).
But that means they are in favor of being on time. So they like one thing and dislike another.
To " have a thing" is to have a strong opinion.