Basically, it is the qualities, promises or attributes of a product that define it as "Me-too", not ad agencies.
Briefly, a "Me-too" product is one which has the same utility and claims the same benefits as the leading brand in the category. For example, take two ball pens, both of which claim good flow, and quantity of ink that lasts and lasts. Now imagine one of those pens having been in the market for 20 years and the other one a very recent introduction.
The second one. which is a recent introduction and claims the same promises as the one that has been around many years is a "Me-too" product. In other words, it is saying "I am as good as the other one."
"Me-too" products are never as successful as the originals, and usually illustrate the ineptness of the marketers handling those products.