B'Elanna had an experience, that at the time she very strongly believed to be real, but she's also a person of science. I feel like it would be out of character for her to not have some questions, even after her journey to Gre'thor.
In "Tapestry", when Q tells him, "I told you. You're dead. This is the afterlife, and I'm God," what is Picard's response? "You are not God."
B'Elanna's in a similar situation. She's informed that she's on the barge of the dead, but is that necessarily divine? Perhaps Gre'thor is an alternate dimension, or something like inside of the Nexus. Fek'lhr could be a powerful being, not wholly dissimilar from Q. In "Homefront", Worf claims that Klingons killed their gods for being "more trouble than they were worth," perhaps that was more than just Klingon myth. We know that aliens visited Earth and were perceived to be deities. The Greek pantheon in "Who Mourns for Adonias", Kukulkan was believed to be the Quetzalcoatl as per "How Sharper than a Serpent's Tooth", and even Satan from Christian mythology was the being Lucien depicted in "The Magicks of Megas-Tu".
Given everything B'Elanna should be aware of regarding the nature of the galaxy, blind faith in an afterlife, even one she's ostensibly experienced first hand, does seem like a big ask.