It's actually using very low quality points of your input to recognise the stuff you're saying. It doesn't require much clarity. The problem is the internet connection. Without a proper upstream, and without low latency, siri won't really do a great job.
Siri doesn't have ears, it doesn't need stereo to understand you. I can imagine it helps to gather more data for better results, but "it sounds muffled" makes no sense when that's output, not input. Siri takes your input. it's output has nothing to do with stereo microphones.
Sorry if I misunderstand you.