I watched it through, though I’ve never read the graphic novels. Perhaps that helped me enjoy it more as I had no expectations for it to live up to. It’s not a series I was wild about, but it fit what I needed for fluffy and mildly fantastical light drama. It’s the sort of thing I watch to turn my head off rather than blow my mind.
I can’t really echo the sentiment regarding transphobia. Being transgender isn’t about changing gender, and a key which allows a person to do so cosmetically, isn’t likely making any commentary on being trans. I can understand the concept of changing one’s appearance bears some similarity to transition, but having been through medical transition myself, there wasn’t a single point while watching where I felt that key mirrored my experience, neither did I feel it was supposed to. In the televised version, it wasn’t clear if that individual even had a gender anymore. It wasn’t exactly the dad’s high school friend anymore after the door was opened.
I’ve definitely had other series ruined for me with that specific transphobic trope (one in particular really springs to mind as quite upsetting), but Locke & Key didn’t bother me. I suppose, as with many things, it will read differently to different people, though.