I could add this option to my Variable Inspector Prompt, if you wanted it. It already supports the option to see all variables, and when I display the value of a variable, I check to see if it's JSON and if it is, I display it that way.
It would be simple enough to add a button to filter on JSON variables.
By the way, the way I determine if something is JSON is I first check to see if it starts with "[" or "{", then do a JSON.parse (in javascript) on it, and if it doesn't fail, then then its JSON.
If this is something you would find useful, let me know and I'll implement it.