It is better to do wider reading, as others have said. You need to show evidence that you don't just rely on lecture material, and that you can form judgements based on academic opinions as well as successfully do independent research.
However, I did have one essay that was mainly from academic literature but I did slip in someting that was from a lecture slide (as I couldn't find the info on any other journal/book) and I just put 'acknowledgements to lecture X, by X'. It's best to avoid using lecture material though unless it's last resort if the info can be found elsewhere. Although I don't see the problem with referencing lecture material if there is a strong valid argument which cannot be found from wider reading.