sudo apt install dmenu  j4-dmenu-desktop

And then in .jwmrc

<Key mask="C" key="space">exec:j4-dmenu-desktop</Key>

The python script  mmaker works in Daedalus if we change the first line to #!/usr/bin/env python3

$ ./mmaker -f PekWM    worked.

https://codeberg.org/JWMKit/EXTRAS

• JWMKit_EasyMenu - Python Version. Full Featured Version.

• JWMKit_EasyMenu_bash - Bash Version

• jwmkit_easymenu_cpp - C++ Version

Instructions are provided as a readme for each version include how to add it to your JWM config, and how to compile the C++ Version.

You could also use JWMKit, but I am amusing assuming you were looking for a menu only solution.

EDIT : Fixed silly typo.as pointed out by stargate-sg1-cheyenne-mtn

I wonder jwm and pekwm are the ones that dont follow the standard.

It is because jwm and pekwm is a very simple wm. It is like constructor, a.k.a. do it by yourself. By the way, you can try  menumaker  https://menumaker.sourceforge.net/

In XFCE installing a programs like Hexchat , Firefox i can see them in the app menu .
If a switch to a wm like pekwm or jwm i can see a Debian menu with some apps but i neither Hexchat or Firefox are in the app menu.

I know there is a related standard Desktop Entry Specification.
I wonder jwm and pekwm are the ones that dont follow the standard.