How to create a mapkey that toggles a custom layer

Short answer is no. In order to "toggle" using mapkeys you will need to create two versions of the mapkey and call them on the fly within Creo using config.pro files or mapkey.pro files if in Creo 11+.

Refer to the video demonstration and sample code posted in this previous thread which will explain what needs to be done to get a pseudo toggle to work. It is not a true logical toggle as the current state is never known but in practice this will not matter.

Solved: Mapkey Toggle Grid (HIDE Grid / SHOW Grid) - PTC Community

You will need two versions of the mapkey saved with the same mnemonic to two files to be read in on the fly. One version will show your layer, and the other version will hide your layer. Each of these mapkeys must include code that redefines what the mapkey does (show/hide). Below are the mapkeys used to enable the glow function shown in the video. The toggle action is completed by changing the setting of config option "enable_selection_glow".

!**** Toggle function for selection glow
enable_selection_glow	no

!**** mapping mapkey to turn on glow
mapkey $F4 @MAPKEY_LABELGlow Toggle;%.ifcp;\
mapkey(continued) ~ Input `file_open` `Inputname` `D:\\ProE Standards\\Config Calls\\on_glow.pro`;\
mapkey(continued) ~ Command `ProFileSelPushOpen_Standard@context_dlg_open_cmd` ;\
mapkey(continued) ~ Activate `ribbon_options_dialog` `OkPshBtn`;

!**** Toggle function for selection glow
enable_selection_glow	yes

!**** mapping mapkey to turn off glow
mapkey $F4 @MAPKEY_LABELGlow Toggle;%.ifcp;\
mapkey(continued) ~ Input `file_open` `Inputname` `D:\\ProE Standards\\Config Calls\\no_glow.pro`;\
mapkey(continued) ~ Command `ProFileSelPushOpen_Standard@context_dlg_open_cmd` ;\
mapkey(continued) ~ Activate `ribbon_options_dialog` `OkPshBtn`;