The professor that taught me statistics dealt with this basic problem, which is that students would come into the course thinking it was like any other "narrative" style of course (like a history class), but the truth is it was a math class that was not listed as MATH 3XX.
Part of the job of a student is to pick courses, ordering them and balancing them into a semester that allows them to handle all of their classes. If they do not have the information they need to balance their schedule (as some classes are naturally heavy loads and others are light), they cannot do their job. So here's how I learned to help them do their job so that they would feel that you were doing your job.
Reset Student Expectations
On week 1 or 2, assign a special test. Include absolutely the most challenging math you expect them to do for the entire semester (not hard for the sake of being hard). Make it absolutely clear that this course does in fact require previous knowledge and ability, and that this test is designed to do just that - test their readiness for this class. You can make it an in-lab assignment and just give credit for doing it - you can give them the answer key and let them grade themselves.
Clearly explain to students that if they cannot complete this test satisfactorily, that is information for them to decide if they are ready for this class. When I ran this lab session I personally explained to students that if they weren't willing to spend at least X hours per assignment per week, they would be unlikely to do well and should reconsider their plans now while they can still re-arrange their schedule. I had a few students drop by the next week, but then everyone who stayed stuck with it all semester.
Don't Make Students Feel Dumb
One of the great dangers of being an expert and a teacher is that you become very distant from that point in time when the material was truly difficult for you, and you lose some of your understanding of what it was like to not know. You are also likely to have an IQ far beyond the average, as well, so you might not ever have experienced this material as difficult - which makes it even harder to avoid this mistake.
I personally remind myself of the time where I had scrawled out about 8 pages of hand-written calculations and equations to solve a factorial analysis of variance by hand, because my professor said it was important. If someone at that moment had looked at what I was doing and even vaguely implied "oh, that's mostly just basic algebra, that's not so hard" I would have stabbed them in the neck with my pencil right then and there. (The pencil was dull and my wrist was tired, so they would have probably escaped serious injury. But still!) Maybe it wouldn't have been hard for them, but it was a challenge to me.
With your advanced understanding of mathematics, you might unintentionally be sending your students a message along the lines of, "oh, this math is pretty basic first year stuff, you shouldn't have any trouble with it". Math is often hard and time consuming and mind-bending and forgotten quickly, even if it is important, and to imply otherwise is insult your students intelligence and character. This will likely result in them not liking you, and worse - they may not learn as much from you as they could have.
Be honest, but take care to honor their struggle with fundamentally non-trivial material. Students appreciate "I know this may seem hard, but you can work through it" more than "this is easy, work harder".
...and make sure the material Is Really Relevant
Yes, math is important and sometimes being able to do it by hand is even important. But I've had teachers include complex material based on the idea that it would be on X industry test that was 3+ years away and that I would not ever be taking, and could otherwise be looked up in < 10 seconds if I really needed it. Sure, now if I get stranded on a desert island I'll be able to calculate proper binary subnet masks so I can build a complex routed network for all my coconuts, but otherwise I'm still a little annoyed I spent hours on that mess when I could have spent it on my research projects.
Motivating the material with a calculation the computer can't solve but they can do it by hand, if such an example exists, can help. It may also be helpful to really drive home the point that if you don't understand the underlying math you'll click stuff that is laughable and makes no damn sense, but the software didn't know any better so it did what you asked anyway.
Still, make sure you really aren't including tough material (according to the students) just because you really think its cool and is technically somehow applicable, but not really necessary or very valuable to the student. Those topics might be a perfect fit in another course - just not this one.
I think you'll find that if students are made to understand what will be expected and necessary upfront, you empathize and honor the challenges your students deal with, and you pair down the material to what truly best serves students, you'll find that not only will students like you more and rate you more highly - but they'll also learn more and the class will be more fun to teach, too.