Was Mikado ever imported to the states? Here in New Jersey its a totally unheard of brand.
Usually when you find something unkown like this its pretty low end from any one of the many discount chain stores.
I suppose this had to have some riding time on it but most likely adult riding, there's some ever so slight signs of road dirt on the bottom bracket and under the fork crown, but nothing that don't wipe off with a damp rag.
I pulled the wheels and started checking bearings tonight, everything was perfect, clean green grease in both hubs, the bottom bracket, and headset. Even the seat post tube was greased inside the frame.
I am most surprised to see that there's absolutely zero greasy spots anywhere on this thing, even the freewheel cogs are immaculate down in between the teeth.
The chain is so clean that your hand only gets a bit oily touching it. If it was ridden, it wasn't more than a dozen miles or so. Even the old vinyl bar tape is still perfect.
It does have a steel seat post, but its only slightly longer than needed, with not much room for raising the seat, maybe an inch or so.

There's a pretty significant resemblance between the Mikado and Miyata badges but I don't recall any Miyata bikes from Taiwan back then?

A local shop owner here said it looked like one of the Raleigh imports from the late 70's to early 80's by Merida but I see similar points on that compare to many different brands, so I can't say any one detail would conclude to who made it. Its probably got the most in common with the late 70's Letour or Traveler type bikes from Schwinn, or a Nishiki Sport, both of which I believe were Giant built bikes, but the location of the serial number is wrong for a Giant built bike.
Every Giant built bike I've seen had the number on the rear dropout. This one is on the rear/lower side of the BB.