No. Firefly and BSG didn't have aliens and that worked out okay.
I don't think a space opera even needs humans. Just a spaceship and somebody to pilot the spaceship. Could be sentient robots. Maybe the spaceships are intelligent and don't require pilots.
All the characters in the space opera are stand-ins for humans anyway. They can be aliens, robots, AI, you name it.
That's just then thing, "aliens" are just a stand in for "the other" or as a reflection of an aspect of humanity. BSG didn't have aliens, but it did have cylons.