I've watched a lot of TV over the years, and I've developed some theories as to why certain people are portrayed a certain way in certain commercials. They may be common theories, and the may be total hooey, but I have thought about it before ever since I heard a comedian (Steve Byrne, perhaps?) bring it to my attention. For the sake of debate, and the fact that I haven't made a very long post in a very long time, especially one at the top of a page, I'll bring my speculations to your attention.
I wouldn't use "wimps" to describe most of the men on these commercials, but I do see what you mean when you say that white guys are often portrayed as stupid on commercials. However, I don't think racism has much to do with it. I agree with it being based on who the target audience is, and I would go as far as to say that white men are portrayed as stupid because they are the target audience.
In most commercials I've seen, guys, white or not, are portrayed one of two ways. There's the more traditional style where the man is portrayed as more "manly" and robust, and then there's the portrayal of the more "stupid" man. In the former, he is almost always an attractive, fit male who uses the product being advertised the way it's meant to be used. These are the "be this guy" commercials. These commercials are effective, but some guys, like myself, don't find them very memorable. The "stupid" guy commercials, on the other hand, usually feature a less-attractive, often out-of-shape guy who, at least from what I've seen, is doing something incorrectly and is the butt of some joke. These are the "don't be this guy" commercials, and they are meant to be humorous and often appear in the middle of comedic shows. I fall under the category of men who respond best to humor, even if I never buy any of the products I see on TV, so I am more likely to remember one of these commercials than a "be this guy" commercial.
Most guys watching TV are going to relate somehow to both the "be this guy" guy and the "don't be this guy" guy. If white males are the target audience, they are going to relate most to a white version of both of those guys, even if it's subconsciously. If an average-looking white guy is watching a commercial on TV, and he sees one featuring a very attractive black man and one with a very attractive white man, he knows he's never going to be a very attractive black man, for obvious reasons, but something in his mind may tell him upon seeing the very attractive white man that he could look like the man in the commercial. The idea that an average Joe viewer could look like or be like an attractive man in a commercial is essentially the idea behind the "be this guy" commercial, and if the average Joe is white, so is the man in the commercial.
The same applies with the "don't be this guy" commercials. Joe is watching TV, and he sees a guy on a commercial whom he perceives to be less attractive and/or less intelligent than himself, so right away he is glad he is not that guy (it doesn't necessarily occur to Joe that the man is a paid actor). If Joe is watching his favorite TV comedy and one of these commercials pops on, it would not be a stretch to think that Joe would also be in the mood to laugh at whatever Mr. Don't Be This Guy says or does, or at least recognize the humor. At the very least, if he's paying even a little bit of attention, he'll think "That's stupid" or "Is that supposed to be funny?" or something along those lines (I've experienced all of the above from different
"don't be this guy" commercials), but he'll at least remember the commercial from having thought about it, and he'll associate that product with that commercial, and/or vice-versa. He's now still more likely to buy the product than if he found the commercial boring and hadn't given it a second thought.
So do "be this guy" and "don't be this guy" commercials exist when the target audience is not white? Of course they do. They aren't as common, of course, but you will see them if you keep your eye out for them. There are also, of course, commercials that combine elements from the "be this guy" and the "don't be this guy" commercials, including the trending "be this guy, but don't be this guy" commercial, where the guy does something funny and/or stupid with the product being advertised.
I would conclude that, yes, some white males on American commercials are portrayed as stupid, while some are not. If you remember the commercials where they are portrayed as stupid, you probably respond well to humor, and are therefore, whether you know it or not, part of the company's target audience.
As for commercials that target females, there are an entirely different set of rules for those that I haven't yet figured out.