Mono has one channel. Stereo has two channels that can carry different sound. A single spoken voice does not automatically need a stereo recording. The useful question is what the microphone and the episode actually contain.
Inspect the source
A single microphone may produce one useful voice channel. An interface can also place that microphone on only the left or right side of a two-channel file. Listen to both sides separately before calling the file stereo.
If one side is silent, do not publish the result with the voice available only in one ear. Convert or route that voice appropriately in the editing workflow, then check the exported file through headphones. Keep the original file unchanged in case the channel decision needs review.
A stereo music track is a different case. Its left and right channels may be part of the arrangement. Combining them without listening can change the music, so keep the voice decision separate from the music decision.
Match the delivery choice to the episode
For an interview made only from centered voices, a mono delivery file may be suitable. An episode with meaningful stereo music or location sound may benefit from stereo. Check the current requirements of the publishing destination rather than choosing solely by file size.
Export a short sample in the chosen format and test it on both headphones and ordinary speakers. Confirm that every voice is present and easy to hear. Write the channel choice in your project notes so another editor understands why a two-channel source became a one-channel voice track, or why the final episode stayed stereo.