According to the creator of Darkwing Duck, Tad Stones, the show exists within an alternate universe from Duck Tales.
“Because Launchpad appeared in DuckTales and we used Roboduck as the Superman character, the hero who gets all the glory as opposed to Darkwing, fans try to connect the two realities. They are two different universes in my book. We work in the alternate Duckiverse,
Darkwing Duck and DuckTales Are in Separate Universes and This Is Not Okay