As someone who draws, I'd say it's important to balance the both.
You can't create good manga if the topic is not at least somewhat interesting to you, but you also have to work with your editor, as well as challenging yourself with themes and settings.
Sometimes editors are just wrong. The strongest works is when the artist is very passionate about the theme, at the end of the day, you'll have to draw dozens, hundreds, maybe even thousands of pages with these characters.