Make up can only hide your most basic flaws but cannot work like magic and make you look something you are not. There is no bad thing as not being prepared to give your best. Lack of preparation can never be hidden whatever the tactics you use. The agencies can very well recognise when someone is not at all ready.
If you trip while walking on the runway, not have enough words to complete your sentence and fidgeting while answering even a simple question, you are in for much trouble and greater disappointment. Filling the room with awkward silence is not something you can afford if you need a good agency to hire you.