Can the Fiora legend buff a unit if it's already a 5 might card?

No, Fiora (Grand Duelist) cannot trigger her ability when a unit that is already at 5 or more Might (already Mighty) receives a buff.

According to Rule 709: "A Unit 'becomes Mighty' at the moment its Might changes from being less than 5 to being 5 or greater."

As clarified in RiftJudge FAQ #9584, if a unit is already at 5 or greater Might, it is already considered "Mighty" (Rule 708). Increasing its Might further does not change its status from "not Mighty" to "Mighty," so it does not trigger Fiora's Legend ability.