While I appreciate the idea, it's so primitive that using draw.io (lucidchart) or miro or basically any other diagramming tool with a grid is going to match and exceed the need.
Agreed, my group uses Google Jamboard and for this to be more useful it would at least need to add tokens for all the standard classes/generic baddies as well as replicating pretty much all the other Jamboard features, especially placing images as backgrounds (none of us wants to redraw a map when we have a perfectly good jpeg from the manual already) and something like the laser pointer to aid communication.