It's certainly not a folk-song in the 1954-Definition sense

As Richard Bridge is constantly reminding us, 1954 Folk is not about a particular style of music, but about a process of adaption and adoption. As it says in the 1954 Definition : The term does not cover composed popular music that has been taken over ready-made by a community and remains unchanged, for it is the re-fashioning and re-creation of the music by the community that gives it its folk character.

Given that it has been changed, refashioned and recreated, I'd say I Don't Like Mondays is most certainly a folk-song in the 1954 Definition sense!