I think it is the other way round. People criticize it because "How to make friends" is the first and more prominent part of the title, while the book is not primarily about that. "How to influence people" fits the content much better than "How to win friends" but "influence" isn't the most appropriate word either and I guess that would not have sold the book nearly as well as its actual title.

Apart from that I think there is nothing to criticize about the content, it might be trivial to a degree, but it is solid and ethical advice, if I remember it correctly.