Unless you do a full installation it will not be a Linux file system, but
likely FAT 32. The reason is that when unetbootin or Ubuntu's Startup Disk
Creator to install from an ISO it does not change the file system. It uses
what came with the usb key which is not likely ext. You can even create a
bootable Linux usb key from Windows by following instructions at
pendrivelinux.com.

When you do a full installation then it would need to create a Linux file
system and that is very different situation and I agree that Windows would
not be able to read it. It all depends how you create it and what kind of
usb key it is.