Is emunah peshutah a good thing, or should I aspire to understand the reasoning behind mitzvos and minhagim?
The answer to this depends largely on personal disposition. Emunah peshutah is certainly a “good thing,” and it is something worth striving for. However, most people will be able to forge a deeper connection with mitzvos if they know something of the rationale and depth behind them. Of course, there are many levels of this understanding, from the basic rationale of mitzvos given by the Chinuch to the deepest secrets of kabbalah, but the very fact that rishonim dealt with such matters demonstrates their importance.
Having said this, deeper knowledge of mitzvos is important for enhancing our perception, and for deepening our performance of the mitzvah, but does not replace emunah peshutah, which remains the basis for our fulfilment of mitzvos — we do not fulfill the mitzvos because of our understanding, but because of our emunah pesutah. The understanding we can glean (which is, of course, extremely limited) only serves to inspire us and to enhance the depth of our performance.