Economy Class tickets don't have lounge access included in them as a feature of the ticket itself, in the same manner that Business Class tickets, until recently, did, so there isn't the same general expectation that Lounge Access will be granted when issued with an Economy boarding pass, as there is when you are issued a Business boarding pass.

So there isn't as much to gain from marking an Economy boarding pass as "Light", whereas marking a Business boarding pass as "Light" - to alert lounge dragons - would seem to make more sense.

That said, if boarding passes were printed with legends like "Light Fare" or "No Lounge Access", then I think we would have heard about it in one of the various Business Light threads by now....though I haven't travelled on a Business Light fare yet, and I guess that most of the people posting here that may have done so get lounge access anyway via status...