Can I use Fiora worthy ability in combat?

Yes — Fiora, Worthy's ability is a triggered ability ("When a unit you control becomes [Mighty]…"), so it can go on the chain during a Combat Showdown.

The Main Phase / Open State restriction (Rule 381) applies only to activated abilities. Triggered abilities fire whenever their condition is met, including mid-combat — e.g., a unit reaching 5 Might from a buff, a Might pump, or Shield kicking in when it's declared as a defender. Resolving it during combat is fine; you just pay the [C] and ready the unit.

Two caveats: the trigger only sees an actual in-play Might change to 5+, so a unit that enters play already at 5+ Might won't trigger it (Rule 709 / FAQ #5835); and like any trigger it uses the chain, so opponents get priority in response.