If a unit is mighty at a might of 5 and plays sacrifice as a reaction, if I stupefy would his sacrifice fizzle?

To answer your question, we must look at how costs are paid in Riftbound.

According to RiftJudge FAQ #9515 and Rule 357, the process for playing a card (including costs) happens before the spell is even added to the chain, and therefore before any player has the opportunity to play a [Reaction] to it:

1. Announcement: The active player announces they are playing Sacrifice.
2. Payment of Costs: The player pays the energy cost and the additional cost. To pay the additional cost of Sacrifice, the player must kill a friendly Mighty unit (a unit with 5 or more Might).
3. Completion: Once that unit is killed and sent to the trash, the cost is considered paid. Only after the cost is fully paid does Sacrifice become pending on the chain.

Ruling

No, your Stupefy would not cause Sacrifice to fizzle.

By the time you have priority to play Stupefy as a [Reaction], the friendly unit has already been killed to pay for the cost of Sacrifice. The unit is already in the trash, and Sacrifice is already waiting on the chain to resolve. You cannot target or affect the unit that was sacrificed because it is no longer on the board.

Even if you were to use Stupefy on the unit before your opponent declared they were playing Sacrifice (i.e., when you had Focus in an Open state), your opponent could simply choose not to play Sacrifice at that time, or they could target a different Mighty unit if they have one. However, once they commit to playing Sacrifice, the cost payment is instantaneous and cannot be interrupted.