Art: There's not much art per se, but the use of color is easy on the eyes. This is a better approach than trying to do too much.
Writing: Given that this pamphlet is really all about the writing, it probably could have been edited more carefully, and I think even within the "Old West" vibes you're going for, there could have been more thought to developing the Old Man Johnson character beyond just an off-the-shelf "prospector with a ghost story" archetype.
Some errors in the text: "Barrow's binary star partialy eclipse" ; "Wearing and whatever the victim had with them in life" ; "tearing hear apart" ; "his wrinkeld, grisled face"
Game Design: I think it's good for the jam's theme to focus on the myth, and I like that you decided to just focus on your monster rather than forcing a scenario around it if you weren't feeling inspired to make one. That said, the ghost story is a lot of text to just make players sit through while the Warden reads aloud. I have tried to run games with bits of exposition like this and I find the players start to get bored if it's more than a couple of paragraphs. They want the game to be interactive.
Theme: You made an in-Universe Mothership myth and also used the jam setting in a way that's essential to the myth. Good job.
Utility: I think you could have pared down the Old Man Johnson narrative and used the space to flesh out the monster's mechanics better. If the pamphlet is just going to be a monster that can be used in other adventures, it deserves more than one little stat block for each of its forms. Also, is "Where the myth derives its name from" a placeholder that you forgot to fill in later? The only mention of the name in the story is "Welp, that was the Iralos Myth."
Favorability: Conceptually, I think you did something cool with the binary star. However, if I was going to be buying a pamphlet that's selling me a monster, and is forgoing both art and detailed game mechanics in favor of a lot of exposition, that exposition had better be extremely compelling and at a professional level of polish. I think Johnson's story is something most Wardens could just ad-lib.