Does Breakneck Mech deflect Thousand Tail Summon effect?

No, Breakneck Mech does not cause Thousand-Tailed Watcher's "When you play me" ability to trigger Deflect.

According to RiftJudge FAQ #8483, #4229, and #4422, the Deflect keyword only imposes an additional cost on spells and abilities that "choose" (target) a specific permanent.

Thousand-Tailed Watcher's ability ("give enemy units -3 might this turn") is a blanket effect that applies to all enemy units simultaneously. Because it does not require you to select or "choose" specific units, it does not trigger the additional Power cost from Deflect.