can we downgrade iPad mini back to iOS 6?

No.

Tap on the link below to my iDevice/iOS performance/optimisation user tips link.

Slow iPad on iOS 9, 10 or 11! - Apple Community

Try all of these tips and leave each tip active together. Do not simply try every tip individually, expecting just one tip to be a performance “cure all”.

Many of these tips are designed to work together to help improve overall iOS/iDevice performance.

Good Luck!

You cannot downgrade.

iOS 6 is now six years old! You cannot even downgrade to iOS 12.1.3, which was released a month or two ago.