If a spell is countered, would Diana No Longer Human still get plus 2 might?

No, Diana, No Longer Human will not receive the +2 Might bonus if the spell you play is countered.

Reasoning

According to RiftJudge FAQ #8742 and RiftJudge FAQ #8945:

1. Definition of "Play": In Riftbound, a spell is only considered to have been "played" when it successfully resolves on the chain.
2. Effect of Countering: Per Rule 425.1.b, a card that is countered is not considered to have been played.
3. Trigger Condition: Diana's ability ("When you play a spell, give me +2 [S] this turn") requires the act of playing a spell to trigger. Because a countered spell does not resolve and is cleared from the chain (Rule 425.1.a), it fails to meet the requirement for triggering her ability.

Sequence of Events

1. Playing the Spell: You place your spell on the chain.
2. Reaction: Your opponent plays a card to counter your spell (e.g., Defy).
3. Resolution: The counter resolves, negating your spell and clearing it from the chain.
4. Trigger Check: Because the spell was countered and did not resolve, it is not considered "played." Consequently, Diana’s triggered ability does not go on the chain, and she does not receive the +2 Might bonus.