It's grammatically correct and a perfectly natural thing to say (I personally am much more likely to say "What an adorable..." than "What an adorable couple of...").

What is the plural form of adorable?

adorable (plural adorables)

Is adorables a word?

Adorable means lovable, sweet, and childlike. The adjective adorable is mostly used to mean "cute," when someone is describing something sweet or charming, like a baby or a pretty dress.