Does Fiora Victorious become mighty when played on Trifarian War Camp?

No, Fiora, Victorious does not "become Mighty" when played directly onto Trifarian War Camp.

According to RiftJudge FAQ #5835 and #9442:

1. Trifarian War Camp has a passive ability that grants +1 Might to units at that location.
2. When you play a unit (such as Fiora, Victorious) directly onto Trifarian War Camp, the +1 Might bonus is applied immediately as the unit enters the battlefield.
3. Because the bonus is applied instantly upon entry, there is no moment where the unit exists on the battlefield with its base Might before becoming 5 Might; it is 5 Might the instant it arrives.

As defined in Rule 709, a unit "becomes Mighty" only at the moment its Might changes from less than 5 to 5 or greater while it is already a game object in play. Since Fiora is already 5 Might the moment she enters the battlefield, she does not "become Mighty" while on the battlefield, and therefore does not trigger abilities that specifically care about that event.