When you activate a monstrosity ability, the creature becomes monstrous after the monstrosity ability resolves
Of course, if you're putting it in your deck, it means you've taken that into account and are probably running it in decks that look to Donate it to opponents with cards like Zedruu the Greathearted, Stiltzkin, Moogle Merchant, and Iroh, Tea Master
If you really want to boost the +1/+1 counters subtheme in the deck (going for that Twenty-Toed Toad/Simic Ascendancy win, eh?), then throwing in Bill and the Brawler give that plan an immediate shot in the arm that can raise the viability of those win conditions right quick
Typically these require zero-mana artifacts to pull off, but Green Goblin lets you be a little more flexible
Brawl Not Legal Vintage Not Legal Timeless Not Legal Commander Not Legal Pauper Not Legal Oathbreaker Not Legal Penny Not Legal Dual Doomsuits Artifact Contraption Whenever you crank this Contraption, each time a source you control would deal damage this turn, it deals double that damage instead