70's Yui is not amused. She'll never be amused, no matter what you do.
Some say 70's Yui was born from the unholy combination of string and G-elements. Others believe her to be the unfortunate victim of some far off Kozuki experiment gone awry, since apparently not even being stationed in Alaska can save you from getting flung back to the far off, far out decade known as the 1970's.
All we know is that she's there, and she's not amused. Probably helps that a lack of Yuuya in the 70's keeps her from turning into a walking panty flood.