A portion of a Gamasutra interview with dev Mattias Dittrich...
GS: How did you come up with the concept?
MD: Honestly there was no "concept". I experiment with visual effects a lot, and came across the visual style of GoNNER almost by accident. I was tweeting about the process and got contacted by Martin Kvale (sound designer). Martin thought it looked interesting and saw the potential in giving it an unexpectedly cute and squishy soundscape. Since then on, the design process has mostly been us discussing what we like and dislike in video games, and trying to steal all the cool ideas while leaving out the things we dislike. A month or 2 into the development Joar Renolen got involved in the project to compose music.
Something that's been central to the whole project is the three of us always trying to challenge each other. Joar said he's comfortable making one song per month, so Martin asked him to create five in a single day.
It turned out very chaotic.
GS: GoNNER features a cute art style that almost hides its vicious difficulty. How did you come up with this style? What drew you to this silly, charming look?
MD: I don't really enjoy the process of animating, so if an animation takes more than 10 minutes to draw, I come up with a way to make it simpler. For instance, Sally, your space whale friend, was originally supposed to be a little creature living in a house, I struggled with drawing a nice looking house, and ended up drawing a huge face instead. In fact, a lot of the design process for the game is based on that "if something is boring/tedious to create, how can I make it more interesting for myself?"