No, it's not appropriate. Sexual experiences are private matters between two people. Sharing such stories without the wife's explicit consent is a violation of her privacy.
I don't think they are common. Cuckold relationships in general are not mainstream, and when you add the factor of the wife being a bbw (big beautiful woman), it narrows down the scope even more. Most people tend to have more traditional relationship models, so these stories are more of an outlier.
It is completely inappropriate. Such stories are not only disrespectful to others but may also violate the law in some cases. We should promote positive values and avoid engaging with or spreading this type of inappropriate content.
Taboo stories involving sexual content are not appropriate to discuss in a general context. We should focus on positive and respectful topics such as body positivity for plus - size individuals in a non - sexual way.