It’s really a spectrum of bad taste. When I grew up people were selling “Dead German Tourist” shirts in Miami, but this isn’t really a “hurr hurr” South Park take on their sad lives and ultimate death.
It’s a reference to history, a detached honoring of their “mission”, and a tribute, in my book.
I don’t see myself as ever encountering someone in the wild who was related to the cult, but if I do, I would at least be able to explain my interest in the history and let them vent, apologizing if necessary.
The interest in cults and brainwashing is very BoingBoing, the execution for something like this is very individual.