I too was looking to toggle that setting off by default, but it appears there is no way to do that.
To not have to click the button myself everytime (which I tend to forget) I created this short AutoHotKey (AHK) function:
; Maximize MobaxTerm to fullscreen without stay on top
#IfWinActive, ahk_exe MobaXterm.exe
F11::
Send {F11}
Sleep 750
SetControlDelay -1
ControlClick, X150 Y10, ahk_class TFormDetachedTab,,,, NA
return
#IfWinActive
When F11 is pressed (the shortcut to make a tab fullscreen) AHK checks if MobaXterm is the active window. If it is, it presses F11, waits 750 ms, removes the click delay and presses the button at window coordinates (150, 10) which is the "Stay on top" toggle.
The coordinates (150, 10) might differ in your case (likely dependent on screen resolution), so use AHK WindowSpy to identify the correct position.
Also note that the 750 ms delay is something that might need tweaking. The toolbar with the button needs to be visible to be clicked, and this delay is short enough for the toolbar to still be visible before auto-hiding.
A longer delay (e.g. 1000 ms) meant that the toolbar was already hidden.
A shorter delay (e.g. 300 ms) resulted in the toolbar not being available yet.