Your asking for it to be a top level menu option right? I don't know...
We already have plenty of options to get there. Not sure if adding it to a top level menu is really all that beneficial.
Compared to:
Add a hot key (one key press)
Add a toolbar button (one click)
<right click track> Edit (Tag Inspector is part of the edit tags dialog)
I am not completely sold on the idea but I suppose, depending on the specific use case, I might be swayed one way or another...
If someone just didn't see it in the edit tags dialog then I feel they did not look for it very hard as that is the logical place it should be.
Edit:
But I am of the opinion it should be in MusicBee menu >> Tools >> Tagging Tools.
For some reason is isn't listed there...