Dumb simply means random, or at least not fundamentally predictable.
All luck is dumb.
There's no good luck, there's no bad luck.
Most people will go through their lives as never win the lottery.
Most people will also probably never get hit my lightning.
Unless you're one of those people that believe the universe is a simulation and every last atom is already presdestined to follow a single path from start to finish, luck is just random chance, happenstance and coincidence. Otherwise, luck could be quantifiable given enough processing power to copy our simulation down to the most perfect imitation. It could be possible. Highly unprobable, though.