Apart from knowing specific languages (java, javascript including react, css), it's definitely helpful to have a good understanding of basic (object oriented) programming concepts. Think of things like inheritance, normalization, polymorphism, database design, etc. While Mendix will handle a lot of it in the background, it's still helpful to have at least a basic understanding of what happens in the background, as it might help you with performance, reusability, data integrity and more.