Page 3, いっぱい – にほんごたどく
I think that, in the second sentence, ナナちゃんは、かわいい, the dog is just making a statement, ‘Nanachan is lovely’, and in the first sentence, ナナちゃん、かわいい, he is imagining that he is directly talking to her ‘Nanachan, you are lovely’.
Am I right, or both sentences mean the same?
No difference in general but depends on the context. In the second sentence, the は is just left out, so it’s exactly the same. Doesn’t necessarily mean he’s talking to her at all.
That said, in casual conversation, sometimes leaving out certain particles can make a difference. は is sometimes used for contrast, other times it’s there to topicalize a term. So in certain cases it does change the emphasis. In the particular example you gave, even though they mean the exact same thing, I feel the first sentence is much more emphasized. You’ll hear the second sentence much more often.