I'm not sure what you mean by Midgard is Asgardian. Midgard is its own realm. Asgard is its own realm.
Other than their skin color, which the Atlanteans share with other MU races, I don't see the connection with the Frost Giants. IIRC, Atlanteans worked in the Red Skull's spell because the Atlanteans are more "magical" than air breathers. They live in one of the most magical cities on the planet. The blood of Atlantean royalty is literally a magical ingredient. It was used to open portals / magical gates. It was used to destroy all the Atlantean vampires. And the Atlantean ruler is a conduit for all Atlantean magic.
Would Neptune be taking on Frost Giant relatives?
All that said. Jason Aaron had Namor on a War of the Realms poster, for some reason, so he may be he's getting ready to make the Atlanteans Asgardian.