Does Ballista trigger Deflect?

Yes, using the ability of Iron Ballista triggers Deflect if you choose a unit with the Deflect keyword as the target.

According to RiftJudge FAQ #1409:

• Iron Ballista's ability ("exhaust : Deal 2 to a unit at a battlefield") requires you to select a specific unit to deal damage to.
• Because this ability requires you to choose a target, it triggers Deflect.
• If you choose a unit with Deflect, you must pay its power cost as an additional cost to activate the ability.