tinycore already has jwm so no need to use jwm extension. The jwm extension is for microcore users, as microcore has no default window manager. The jwm extension has no additional functionality.
If you elect to use an alternate window manager extension, (e.g, flwm or hackedbox) then you do not need to use the desktop=.... However if you have more than one additional window manager extension being loaded then the last one (alphabetically) will be defaulted. You may always override the defaulted window manager by using the desktop=... boot code.