In my opinion, moon palace is first a bildungsroman, then a travel diary.
What I mean by that is, the travel aspect is more so a way for the protagonist to discover himself, it is a tool for self discovery.
He travels to the west for a new beginning.
If it was simply a travel diary, we wouldn't have explored the themes of grieving, dealing with money problems, solitude, being an illegitimate child, facing homelessness in such detail in the first chapter.
The only elements we see that this is a travel diary (for me) is that he'll go to the west and that he has to go from his mom's home, to his uncle's, to an apartment which i would count as traveling.