Technically, the imp isnt necessarily a "real" imp. It isn't the final form of some evil baker or low level mook. Find familiar puts a spirit of your choice (fey, celestial or fiend) into a form. Normally you only pick beasts, but the Pact of the Chain lets you pick other forms. So you could have a fey imp, a fiendish pseudodragon, or a celestial quasit (just kidding, no one ever picks quasit).
I don't think this matters as to what it would look like to others.
There is no Magic that makes a Spirit look like a Fiend. ( Appart from Polymorph. )
It's not like spirits have much of a form to begin with. They are pretty much shapeless in conception and takes shape when they enter the material world.
And yes, there is such magic. That's exactly what Find Familiar does.
