I don't think any book is "too" mainstream, however it is important to note that what you say about what you read is FAR more important than what you read. You could make an interesting point about the Mr. Men books for English Literature or Art and it would be better than just name dropping books.
So, no - no uni will disregard you or dock you points for mentioning a mainstream book. However, if all you say is that you read it, it'll probably be disregarded as irrelevant and will make you seem a very superficial candidate. If you talk about what you thought, what it made you question and your personal reaction, it'll make it more interesting. Because you are mentioning your PERSONAL reaction and thoughts, it will be different to any other people and so will be interesting. However, do not just give a basic summary of the book because that will once again make you a boring, mainstream candidate.
TLDR; it's fine as long as you talk in detail about what you thought about the book, what you did with the knowledge you gained from it and your personal reaction/questions to it (this is true of any and all books)