No, it wasn't Loki. If you stayed past the credits in Thor 2, you get to see Natalie Portman’s character kissing on Thor. But the actress recently revealed that it wasn’t actually her lips in that scene.

Turns out that the close-ups had to be re-shot. Since Natalie was busy doing another movie, they had Hemsworth’s real-life wife Elsa Pataky do it instead:
“It was for reshoots (after the film was finished) and he was working in Hong Kong and I couldn’t get there because I was working on my own film. And so they put his wife in my wig and costume, that’s why it was so passionate. It was such a perfect solution, wasn’t it?”