Hello,
 I just upgraded from FC30 to FC 31.
 it seems that mock's "default.cfg" is still pointing to the FC30 config.

cd /etc/mock/
[az@dell5000 mock]$ ls -l ./default.cfg
lrwxrwxrwx 1 root root 20 Nov 24 12:52 ./default.cfg -> fedora-30-x86_64.cfg

[az@dell5000 mock]$ uname -a
Linux dell5000 5.5.16-200.fc31.x86_64 #1 ...
...
[root@dell5000 ~]# dnf whatprovides /etc/mock/default.cfg
Last metadata expiration check: 1:41:15 ago on Sat 18 Apr 2020 02:52:52 PM EDT.
mock-core-configs-31.6-1.fc31.noarch : Mock core config files basic chroots
Repo        : fedora
Matched from:
Filename    : /etc/mock/default.cfg

mock-core-configs-32.4-1.fc31.noarch : Mock core config files basic chroots
Repo        : @System
Matched from:
Filename    : /etc/mock/default.cfg

mock-core-configs-32.4-1.fc31.noarch : Mock core config files basic chroots
Repo        : updates
Matched from:
Filename    : /etc/mock/default.cfg

thank you for checking.