When Alpha Brawl is played, after the target creature deals damage to the opponents creatures, do only the ones that survive do damage back?
Creatures dealt lethal damage will still deal their damage back to Alpha Brawl's target.
From the Oracle rulings, which you can see on the card's Gatherer page:
Why does it work the way it does? Because, in Magic, you carry out the instructions on a card sequentially, but you also do all the actions as one event, and only check for state-based actions and triggers afterward. "This creature took so much damage that it's going to the graveyard now" is a state-based action.
Note also that all creatures killed by Alpha Brawl will go to the graveyard at the same time.
The ruling is right there on gatherer:
All the opponent's creatures will deal damage back, regardless of whether or not they die.