You have misunderstood why people use the term "extremism".
> Mainstream views can't be extreme by definition.
Of course they can; mainstream views can't be uncommon by definition. Extreme doesn't strictly mean uncommon (not even in a political context), it is also used to mean "high degree", which can include distance from political centrism but can also include, e.g., frustration or flavor of cookie. To give another example, various online "challenges" like the "ice bucket challenge" are extreme but were also relatively mainstream when they were commonly performed and posted online; the term "ice bucket challenge" is still mainstream and the challenge itself is extreme (in fact, the reason it's called a "challenge" is because it is extreme).
Thinking there's too many immigrants might be mainstream (it currently is) but whether or not it's extreme depends on the degree to which it's believed. If it's believed to a high degree (such as "immigration is the worst thing about the capital city of this nation") by a large number of people then it is an extreme mainstream view by definition.
> various online "challenges" like the "ice bucket challenge" are extreme
Ok I think you just have a very abnormal (extreme even?) definition of the word "extreme".
In a political context it literally means "far from the norm". His views are not far from the norm, as much as you might hate that. (I'm not a huge fan either but I'm not going to distort reality to make myself feel better.)
> In a political context it literally means "far from the norm".
No, this is simply what you want it to mean, keeping in mind you're trying to tell other people what they mean with their word choice. Extreme views can be normal and mainstream and typical. There are many normalized-but-extreme views in current mainstream politics.
> Ok I think you just have a very abnormal (extreme even?) definition of the word "extreme".
Pouring a bucket of ice water on your head to bring attention to something is extreme. Like, it's over-the-top and exaggerated. You can disagree but that's kinda moot: someone isn't strictly wrong that it's extreme, you just disagree. You still didn't address the greater point that extreme, as it's being used, is orthogonal to mainstream.
But I guess I can link to a dictionary so you can see that I have a pretty normal (and mild) definition of the word in question. I hope you don't cherry-pick definition 1c, ignoring definitions 1a and 1b, which are, of course, valid.
Maybe 4 is the best definition, seeing as it gives "the extreme political left" as an example usage. It's not obvious to me how "advanced and thoroughgoing" means "not mainstream", though. I wouldn't mind an explanation.
You have misunderstood why people use the term "extremism".
> Mainstream views can't be extreme by definition.
Of course they can; mainstream views can't be uncommon by definition. Extreme doesn't strictly mean uncommon (not even in a political context), it is also used to mean "high degree", which can include distance from political centrism but can also include, e.g., frustration or flavor of cookie. To give another example, various online "challenges" like the "ice bucket challenge" are extreme but were also relatively mainstream when they were commonly performed and posted online; the term "ice bucket challenge" is still mainstream and the challenge itself is extreme (in fact, the reason it's called a "challenge" is because it is extreme).
Thinking there's too many immigrants might be mainstream (it currently is) but whether or not it's extreme depends on the degree to which it's believed. If it's believed to a high degree (such as "immigration is the worst thing about the capital city of this nation") by a large number of people then it is an extreme mainstream view by definition.