Is not even encrypted btw.
Yes, I understood your intend. But as explained before, this would not be effective. Let’s ignore everything I wrote before about the negative impacts this would have in regards of users not understanding the importance of this file:
If you think that attackers were just wandering around on your disk until they stumble upon something that “looks interesting”: This is not how it works. Maybe script kiddies that seek to do some damage act this way, but you are talking about a hypothetical ransomware that specifically targets Cryptomator and “steals” the masterkey file while leaving all other files untouched despite apparently having write access and therefore being able to deploy a “generic” ransomware (which isn’t worth it from a financial perspective to beginn with).
Even if said file had a non-deterministic name, such targeted attacks would still be able to distinguish it from other files by its metadata and structure unless applying steganography to all files (which again is a different field of applications).
All this effort while there is a simple solution to it all: Backups.
Just for the sake of completeness: There are other rationales for having a “non-obvious” name, such as plausible deniability.