Is any card above 5 might considered mighty?

Yes, any unit with a current Might of 5 or greater is considered "Mighty."

According to Rule 708: "A Unit 'is Mighty' as long as its Might is 5 or greater."

Additionally, per Rule 710, units on the board are evaluated based on their current Might, which includes their base Might plus any active modifiers (buffs, debuffs, or attachments) at that moment.