When I play Undertitan do I add the 2 energy from the revealed?

No, you do not add the 2 energy when you play Undertitan from your hand.

According to the official Riftbound rules and FAQs (specifically RiftJudge FAQ #9429 and #8794):

• Playing is not revealing: Undertitan's energy ability, "As I'm revealed from your deck, [Add] [2]," only triggers when a game effect explicitly instructs you to reveal cards from your deck. Simply playing the card from your hand (or any other zone) does not count as revealing it, and therefore does not trigger the energy gain.
• Distinct Actions: Drawing, looking at, and playing cards are distinct game actions that do not count as "revealing" unless an effect specifically uses the word "reveal."

When you play Undertitan from your hand, you only trigger its first ability: "When you play me, give your other units +2 [S] this turn."