does not belong here. It would create a file in the home directory of the package builder machine. That said, files in /home should probably not be created by installing packages anyway. If you really want the file to exist by default, convention is to either put a template in /etc/skel (doesn't really make sense on qubes), or likely preferable: create it on first run of the tool by a given user.
The Makefile install target modifies state of builder machine, not machine where the pkg gets installed.
https://github.com/anoadragon453/qubes-mime-types/blob/a89d12edf75abe494fd8919d1bb1d1b59ee49536/Makefile#L34-L39
(or similar) makes more sense in a %post scriptlet in rpm .spec
https://github.com/anoadragon453/qubes-mime-types/blob/a89d12edf75abe494fd8919d1bb1d1b59ee49536/Makefile#L30-L32
does not belong here. It would create a file in the home directory of the package builder machine. That said, files in /home should probably not be created by installing packages anyway. If you really want the file to exist by default, convention is to either put a template in /etc/skel (doesn't really make sense on qubes), or likely preferable: create it on first run of the tool by a given user.