There are many pros and cons of mass marketing. Unfortunately, the term "mass marketing" is a little bit ambiguous. It could refer to the actual attempts to sell a product to a large market population or it could refer to the advertising of a product or company through media like television, radio, and/or print sources.
A pro of the former is that a company can sell products to a much larger population. This means more sales and hopefully more profit. The larger market also gives the company the opportunity to lower the purchase price for the consumers. The smaller profit margin is then offset by the increased number of sales. A con of the former type of mass marketing is that the products are usually generalized products instead of specific products. This isn't necessarily bad, but it generally means the consumer isn't getting the best product possible in that particular category. Let me use the restaurant The Cheesecake Factory as an example. Their menu is huge, and you can order anything from tacos, to Chinese chicken, to steak. The food tastes good; however, most consumers aren't likely to say that the restaurant makes the best tacos they've ever had. For that, you go to a taco shop. For amazing Chinese chicken, you go to a dedicated Chinese food restaurant.
A pro of mass marketing through advertising is that the industry itself keeps people employed. The advertising industry has writers, directors, producers, cameramen, actors, sound designers, etc. A con of the advertising industry is that it has a tendency to change wants into needs. More often than not, consumers are being sold something they truly don't need.