If you want people to read and understand what you write, yes. Written language is more complex, which makes it more work to read. It's also more formal and distant, which gives the reader's attention permission to drift. But perhaps worst of all, the complex sentences and fancy words give you, the writer, the false impression that you're saying more than you actually are.
For many things I write I aim for my 8-year-old to be able to get a decent handle on it. I also read most of my work out loud to smooth things out because my writing can sound odd otherwise.