Is it necessary for people who program mobile games to also become graphic designers?
NO. NO. NO. NO.... NO... OMG NO.
For games, art is not an afterthought. It is not something you wipe up at the last minute, and be done with it. If programming is skeleton, design is organs, art is skin and hair. It is what make your game a presentable product instead of a barebone concept.
Think about your 3 pillars: Art, Design and programming, each is equally important to the other two. Each should have its own dedicated team, if not team, at least a person. You should have an artist, a designer and a programmer. Because these disciplines require different training, different way of thinking, and each solves its own set of unique problems. You can't cut corners on these. You can't hire an programmer and require her to also design your game, you can't hire an artist and ask him to program your game.
That's not to say cross-field training is unnecessary. An artist who know programming would know the potential technical difficulties for an art asset and try to create it differently, while a designer who knows a little bit of art, would make communication a lot easier if he draw out his level or character design. He might not have the caliber to create the final in game art, but a picture sure beats 10 paragraphs of description.
For small start ups, these responsibility often blend together. Artists needs to know programming, programers sometimes got caught up and have to help out with art. But just because some people are doing it, doesn't mean it is a good practice, and it certainly doesn't mean it is a necessity.
Hire an artist, Gosh. We're not that expensive! A lot of us are more than happy to work with less money if the project is interesting enough. Don't ask your poor programmers to draw stick figures.
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