I'm going to assume that the title is not speaking of graphology, the claim that penmanship is a good indicator of someone's cognitive processes or emotional state. Graphology is pseudoscientific woo that has been widely debunked.
As for how you write, of course it would reflect your thinking process. If you're a modernist writer who incorporates stream-of-consciousness writing, this will obviously be the case. Even if you are not, there is bound to be some level of spontaneity in the stuff you write, which can indicate how you think. For example, some people might be inclined, when forming their opinion on something, to think of concrete examples first, and use induction to draw a conclusion. Others may start with a few premises and deduce their way to a conclusion, and then throw some examples in for good measure. The essays you write will reflect these thinking processes.
Even on a micro scale, certain peculiarities of your writing might reflect how you think, such as your tendency to use language features like pronouns, voice and tense. However, I'm usually sceptical about these claims and I think it is necessary to warn of the seemingly endless stream of false claims about micro aspects of language reflecting, or even affecting, thought.
Let's consider the alphabets of the word 'WATCH'. W stands for words, A for actions, T for thoughts, C for character & finally H for heart. This indicates that all these five are related & function of one affects the function of other. As writing is an action it affects our thoughts & also the other three mentioned above.
I believe, thoughts creates the destiny. What we do is only the result of our thoughts, our thoughts becomes our KARMAS. Not only writing style but also our every act reflects our thinking process. So, i totally agree with this fact. Well, exception are there in every fact but in most cases writing style is indicator of ones thinking process!!
Some people think in language while others think in concepts. Conceptual thinkers are not always as articulate as language thinkers. They have to work harder at putting their thoughts into words. But the more articulate a person is, the more their writing style will reflect their thinking process. There is only a mismatch to the degree that a person is inarticulate. But nobody who uses language is completely inarticulate.
While it may be true that a person's capability to write may be greatly jeopardize by his failure to comprehend certain concepts, it is important that we take into account the group of people who are intrinsically poor writers. Significant thoughts processes may very well be present on their minds. Even if they are unable to present their argument or view in a coherent manner, it still does not dismiss the immense amount of time and effort placed in understanding or debunking certain logic. As such, while a person may be incompetent in writing, it does not devalue their ideas and views. One example of this is Abraham Lincoln. Before he became a great lawyer prior to his role as president, he was unable to convey his ideas effectively though writing. However, salient ideas were definitely inherently present within the mature mind that he had. Language, the very heart of conveyance, is only effective if, and only if, the person has a clear idea of what he has to say. But at the same time, failure to convey a message may also be attributed to the incompetence of one's language abilities. Therefore, writing style may not always accurately reflect the thinking process.