iPad mini model A1455

Your iPad mini2 can certainly be charged, without issue, using a 20W Power Adapter - although for older model iPads, such as yours, there is no technical benefit in doing so. Your iPad mini2 will charge at its maximum supported charging rate when charged from a 12W (2.4A @5V) standard USB Power Adapter.

While you can use a Power Adapter that supports USB PD (Power Delivery), such as Apple’s 20W USB-C Power Adapters, your iPad will only charge at the 12W rate. The mini2 does support USB PD - and as such, any USB PD compliant Power Adapter will default to 5V USB charging.