Indeed, the writing in a dissertaton must be crystal clear. The words must convey exactly the meaning intended, nothing more and nothing less. Each statement in a dissertation must be correct and defensible in a logical and scientific sense. Moreover, the discussions in a dissertation must satisfy the most stringent rules of logic applied to mathematics and science. Writing a dissertation requires a student to think deeply, to organize technical discussion, to muster arguments that will convince other scientists, and to follow rules for rigorous, formal presentation of the arguments and discussion.
Good writing is essential in a dissertation.
However, good writing cannot compensate for a paucity of ideas or concepts. Quite the contrary, a clear presentation always exposes weaknesses. Each term should be used in one and only one way throughout the dissertation. Use strong words instead. They have no place in a formal document.