From the time they first settled in the US, the Amish have been different. They spoke German and eschewed the clothing that was popular. They had specific rules about dress and behavior, like no buttons, which set them apart even before modern technology. They also have their own religion that is a branch of Protestantism. As a result, they'd be going to their own church and hanging out in their own social circles.

Also, has there ever been any anti-Amish sentiment?

Background: I grew up Mennonite (similar to Amish), and I have Amish in my extended family.

Before I go trying to pass off family anecdotes as history, here is a source with more info and examples: http://www.kshs.org/p/kansas-historical-quarterly-mob-violence-and-kansas-mennonites-in-1918/13278

Now, you asked whether there has ever been any anti-Amish sentiment, the answer is yes, particularly in WW1.

Two things to understand about Amish:

1. Amish don't believe in going to war. Conscientious objectors, in other words.
2. Amish to this day speak a dialect of German.

So in WW1, you have Amish and Mennonites who speak the enemy language, and on top of that they're refusing to join the military and go fight. It was not uncommon for them to be viewed as traitors because of it.

There are a number of stories in my family of people getting tarred and feathered, jailed, or even killed. Some of it is no doubt exaggerated with the passage of time, but the sentiment certainly existed for a time.

For WW2, I don't have any sources so I can't say anything definitively...that said, I've certainly been told by my older relatives that the sentiment was much reduced compared to WW1. Why, I have no idea