Undertaker and Mid Card title doesnt go hand in hand at all, it would insult Taker's Legacy.
I imagine some people would've said that about his Hardcore Titl ereign, but he made that shit work.
I would only be for this if it wound up putting a mid card star over huge.
Example:
Heel 'Taker starts feud with current US champion.
UT wins US title strictly to humiliate the face.
Face turns up the heat and takes it viciously back at 'Taker
Face wins back US title and UT moves on to the next feud.
This gives both the face and the US title a nice rub, and its a very temporary angle as it is.