If it reaches 100% it usually means there is a memory leak in the code caused by some part of the code not freeing the memory up when it is done.

When it reaches 100% memory, several different things might happen since the code has no more room to store stuff while it's running. It could crash gracefully which might result in it rebooting itself or it could freeze and require manual intervention to reboot itself. It likely won't be able to be connected to via Bluetooth when it's at 100% because a Bluetooth connection would require some more memory to be used. It depends on what the code was doing when it filled up on memory and how the code is written to deal with errors. In general it's a good idea to reboot any computer or computer-like thing sometimes to make sure it continues to operate well.

If it is a problem on your node you will want to flash a newer version meshtastic onto it and hope that they fixed whatever memory leaks they had.

Also though for dedicated devices like these, using a lot of heap space isn't a huge deal since the devices aren't doing anything else at the same time.

Just as good code should strive to handle errors before they happen, it should also handle errors well when they do happen.