I'm sure this is extremely common. Brilliance and ambition are more dispersed in men (with longer right tails), so if you are a highly highly intelligent and highly highly ambitious man, you are likely to 'marry down' in these respects. Similarly, women are on average more functional and verbally intelligent, and they are relatively more valued for their looks than for personal and professional status. This means that if you are a smart, ambitious, but average-looking woman, you will almost certainly marry down in terms of intelligence and conversation and basic functionality and put-togetherness. Put more tangibly, the average male entrepreneur and the average female nurse are probably both 'marrying down', professionally, in the 21st century.
I can't speak for women, but I don't think most men have much anxiety about this: when you love somebody, you love them, not their resume. I'm a serious racer, my wife is a participatory hobby-jogger who keeps me relaxed and smiling at races. To be honest, I probably prefer this to her being an extreme, over-skinny, masculine competitor. (I'm sorry if that offends the serious female athletes on this site--I'm not saying women *should not* be intense athletes, just that that is not what would attract me to a woman.) One of those obvious but unspeakable truths is that, pretty much by definition, heterosexual men are attracted to feminine characteristics while heterosexual women are attracted to masculine characteristics. So it's the opposite of the truth to assume that a mismatch on certain gendered characteristics undermines a happy relationship.