Smiling can help you feel relaxed which in turn might help you perform better. However, if you smile or laugh too much, it might not be appropriate for the topic you are discussing. Every examiner is trained and monitored closely to be fair to every candidate and to assess your level of English. The examiners will not give you more or fewer marks if you are smiling and laughing. It is best to try and relax as much as you can and communicate in a natural way.