Most small cars are not “friendly” for any mechanic. Due to small spaces things get wedged in and access can be hard. Is it “engineered” purposefully to make it hard for a home mechanic? I don’t think so. Cars have had more stringent emission numbers to meet compared to trucks and that has added to the complexity of cars compared to trucks. The engine bays of trucks are huge compared to cars, making trucks much easier to work on.
My guess is the Fiesta is just as mechanic friendly, or not, as the Yaris. Base your decision on some other criteria, me I like the Fiesta over the Yaris. I have an '03 Honda Civic that needed a simple replacement of the alternator. Should be easy, and on any truck pretty much a snap. On the Civic I had to pull off the power steering pump to get to the alternator from the top, and take off the drivers side wheel to get access to the bottom. This turned a simple job into a body contortionist mess. I paid the $90 for 1.5 hours labor for my mechanic to do the job. It would have taken me much longer, meant crawling around under the car on a jack stand in a very awkward angle. If I snapped off a bolt it would end up at a shop anyway, and that seemed highly possible.
Your home mechanic can still do oil changes, replace plugs, and lots of other stuff. Just sometimes you have to decide whether you want to do a job yourself if access is a problem. As for special tools, this is an issue on lots of cars and I think this is less a problem with Ford than Toyota as a brand.
By the way, the distinction between “American cars, and foreign cars” is pretty much mute now. The Fiesta isn’t any more American than the Yaris. Cars are built everywhere and the Fiesta was made to be sold all over the world, not sure where it is manufactured. Ford is an American Company, but it makes lots of “foreign” cars if you consider the site of manufacture. Most of Ford trucks sold in the US are made in North America, while a much smaller percent of Ford cars are made in North America.