It sounds rather silly, why does there need to be a term?
People like to have a cool label (exactly what BMW exploit with these cars, as do Audi with S cars and Merc with 43/53 AMGs) but obviously they get slapped down quickly by the M purists who refuse to acknowledge the new M Performance models as anything to do with M ownership communities.
'Baby M' just sounds dreadful, more like an infant you'd read about in the news for all the wrong reasons.
M Lite rolls off the tongue a bit easier than M Performance Model though, so it's easy to see why that's relatively popular