There is no question whatsoever that GOOD artwork DID sell games for the PC Engine. Flavor of the month character designers could make or break games back then. Yuna, Startling Odessey. Hell, somebody bought Energy and it wasn't because they had ever played the f*cking thing. It had to have been the box. I would actually say that a huge part of the PCE apeal was the killer box art.
Hell, I STILL buy PCE games just because of the box, and through the magic that is wholesale piracy I can sample everything before I buy. Similarly I will buy and re-buy specific versions of movies, books, etc just to get the cooler packaging from specific releases.
Of course not everyone sees it this way, but many people here only buy US games, and only buy them out of nostalgia. They actually think shitty covers are awesome. My brain doesn't work that way.