That's only your opinion I don't see any evidence of that personally, especially as Lewis is on record as saying he hates doing PR and promotion work, which is what marketable boils down to for the sponsors they ain't gonna pay someone not to promote their products....
As I see it, it depends on the country you are talking about, for instance in Germany Lewis isn't the most marketable driver Vettel is, in Spain Lewis is not the most marketable driver, Alonso is, in Italy Lewis is not the most marketable driver, once again Alonso is, in Japan Lewis is not the most marketable driver, Button is due to his girlfriend and the amount of time he spends there. in Spa Lewis is not he most marketable driver Kimi is and is probably the most marketable driver of the entire bunch in any country simply due to how laid back he is..
I don't think I need to go on...