Does Janna remove might reductions done earlier that turn when she heals?

No, Janna's healing ability does not remove Might reductions applied earlier in the turn.

According to RiftJudge FAQ #560:

• Ruling: Healing does not remove negative Might effects. Healing only removes marked damage from units.
• Nuances: Negative Might effects (such as Stupefy or Smoke Screen) are not considered damage; they are stat modifications or debuffs. Healing specifically targets marked damage and does not clear these modifications.

This is consistent with the general rules of Riftbound, where healing is defined as clearing damage from units (Rule 405.1), and stat modifications (like Might reductions) are tracked separately from damage.