There is already such tool. Official MiSTer SD card installer.
Of course there is, that's why we have this thread here.The card installer is limited and does not work on all PC configurations. I know, nobody stated it is perfect and will work an every possible combination of hardware and OS and I'm not complaining about it. I fully understand and support the idea that if something does not work for me, it's up to me to fix or improve it or create something better. No issue with that.
Back to the topic: I've re-checked the Official MiSTer SD card installer and there is no option to create an SD card image (as I got an impression from your reply that such option exist in the installer). The only option (on my Win 8.1 computer right now at least) is to close the program because (possibly) it did not find the drive to write to - this is only a guess because there is no indication about why it disabled all the visible options. Again, this is not whining, I'm just stating the fact. Btw. running version 188.8.131.52. I also do not expect anybody to fix it for me, I managed to use the installer after some fiddling. But I understand how frustrating it could be for somebody to buy a new DE10nano board to run MiSTer, but have no way how to prepare the SD card, because the installer does not work for him/her.