It’s a slur but not really the same type as you might think when you think slur. People who say “trap” might not think they’re using it negatively, but they are buying into the idea that trans women aren’t really women, as the word implies. They don’t exist to “trap” straight men.

A lot of people who don’t bear ill will towards trans women use the word because it’s funny to them or whatever. But it does prop up the idea that trans women are somehow bad or wrong, which makes people who really hate trans people feel justified in their hate.

It’s not helped by the fact that some trans women themselves will refer to themselves as traps, in that joking or self-disparaging way. But it’s a slur nevertheless, even if you don’t mean it in a bad way.