Not quite as horrific as some of Ito's other stories, considering that there is very little that is uncanny about the mummies or the religious order (oh, they wanna convert people to be followers? What religion doesn't? And they even let the kids escape). Even at the end, where the trio gets lost in the labyrinth and stumbles upon the living members... those are just regular dudes. Like, none of the L O N G L I M B S or W E I R D F A C E S that Ito usually includes. Heck, these guys are Buddhists, even if they are part of an extreme sect. Considering they're still alive, I wouldn't discount the possibility that somebody guided them back out after some amount of awkwardness. For a better Junji Ito story concerning mountains, I'd look to Mountain of Gods: Precipice of the Unknown (Kamigama no Yama: Makyou no Mine)
Alternative explanation: the actual topic is Sayoko's trauma and her trigger concerning people staring at her. The mummies are positively kitsch to anyone who grew up past the 1950s, but to Sayoko they represent a mental obsession she has over people looking at her. Ultimately, this story is much less about the Buddhist sect's strange practices and more about the mental state of Sayoko.
Note: Buddhist mummification is a real practice, although extremely rare and verboten, and really it's just a form of consensual suicide.