MacTex is apparently 4+GB to install. It says on the Lyx downloads that MacTex is a necessary dependency.
Do we really need MacTex?
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It depends on what you mean by the question:
Do we really need MacTeX specifically? - No, you do not need MacTex specifically. You could in theory use other LaTeX distributions or install LaTeX yourself. MacTeX is just an easy and convenient way to get a full LaTeX system running on your Mac.
Do we really need TeX at all? - Yes, you do need a working TeX system in order to produce documents. As soon as you want to render the document, print it, or similar, you will need TeX - and LyX alone won't suffice.