It seems like there are two separate angles to address with your implementation using MATLAB to read COM port values from your microcontroller: reading values from the serial COM port and getting a continuous stream of data. In order to give a thorough response, I will address both these approaches sequentially.
You can import data into MATLAB through a COM port by creating a serial device in MATLAB that acts as a communication channel. In order to create this serial object and read and write data, please refer to the details in the documentation page below.
There are several additional steps you can take in order to receive a continuous stream of data. First, you will want to set the 'IsContinuous' property of the serial object you created in the first step to 1, allowing a nonstop flow of data. If you are interested in plotting the data and having it update continuously, you would want to create a callback function which uses the 'BytesAvailable' property of the serial object to update your plot after a certain amount of data has been registered in the serial buffer. The first link I attached below goes over the valid callback properties of serial objects, while the second is a general overview of callback functions and their implementations.