Whenever designing a booklet cover page, one must keep in mind the audience he is to attend to. If you are to publish something for people to give awareness on a topic they are unaware of or they are less educated off then, you must use simple yet clear words which help in conveying your message to the group of audience you are addressing. Here the use of complicated words will not only make your efforts void but also will be of no interest to the reader.
Moreover, if you are addressing kids on a certain topic the choose images for the cover page that help attracts children, and if you are to write for adults then stop choosing childish images. Reflect maturity in your style.
In the end, it would not be wrong to say that a booklet cover page must not be dull. It should be designed with catchy colors depending on the audience you are to address. It should reflect positivity in the minds of the reader of the booklet. One should always show respect for others while addressing sensitive issues. While designing a booklet cover page for a topic that is a sensitive issue, one must not use images that show harshness towards someone going against it. It should rather provoke the reader to change his mind and move towards positivity.