Fermentation temperature is an important variable to monitor & control for the brewing process: too warm, and your beer will develop solventy off-flavors and headache-inducing alcohols, but too cool, and you’re liable to have sluggish yeast, even risking a stalled fermentation.
For the well-mixed liquid that is your wort during active fermentation, the temperature as read at the exterior of your fermenter is going to be virtually the same as the middle (or anywhere else). I’ve checked using my thermowell and temperature controller (with 0.1°C accuracy), and the two monitoring methods agree as much as is allowed by their different levels of precision.
Please note: I am making no statements about the accuracy of the air temperature surrounding the fermenter compared to the temperature of the liquid in the fermenter. These can potentially differ significantly!