No, “How nice!” means “That’s great!” or “That sounds wonderful!” It’s used to express a positive reaction to what someone said. It does not mean “It was very nice.” Instead, it’s more like saying “That’s lovely!” or “That’s good to hear!” in response to good news.

Very often, exclamatory sentences will begin with the word 'how,' even though 'how' is usually a question word. It's usually used as a reaction to something surprising. For example: "What a beautiful rainbow!"