I bought a mophie 100W fast charge usb-c GaN charger that can charge a device at 100 watts. Can I charge my iPad mini 6th generation at 100 watts?

While you can use the mophie Power Adapter to charge your iPad, your iPad will not benefit from the full available output.

100W is the maximum power output of the Power Adapter. The iPad6 can only charge at its maximum supported charging rate - this being 12W (i.e., 2.4A @5V).

When using an otherwise healthy iPad and Power Adapter, your iPad will come to no harm. I myself routinely use a mophie Speedport 120W Power Adapter to charge all my Apple devices:

• mophie speedport 120 4-port GaN wall charger (120W)

https://store.apple.com/uk/xc/product/HPZE2B/A