Something marvelous is unusual, wonderful, and almost miraculous. Finding a bag of gold would be marvelous by any standard.
Something that's decent or even simply good isn't marvelous. A marvelous thing needs to be really stunning, exceptional, or hard to believe. Marvelous things make people marvel at them — they're amazing and a little shocking, like a miracle. Winning the lottery would be marvelous. Finding out that ice cream is actually good for you would be marvelous. And if you want to make someone feel extra nice, tell them their new hat looks marvelous.