I started my academic career as a high school teacher. As part of earning the credential to teach high school, I had to go through a semester-long "internship" as a student teacher---I taught in a classroom under the close supervision of a veteran instructor. One of the first bits of feedback I got was that I had a tendency to say that a problem was "easy" or "clear". In my own head, it was in the context of a universe of possible problems, e.g. "This problem is easy as compared to this other problem on a similar topic," or "Let's start with an easy example before moving onto harder examples." My supervisor encouraged me to avoid this kind of language, because
- no problem is easy and nothing is clear, and
- telling a student that a problem is easy destroys morale when they can't figure it out.
No problem is easy
There is a great Saturday Morning Breakfast Cereal comic which describes a view of how ideas come about in mathematics, and how we disseminate those ideas. Essentially, modern mathematics is the distillation of several thousand years of thought. The ideas and results were hard-won, yet we somehow expect students to master them in very little time.
I imagine that this observation is true across disciplines. For example, I can't imagine trying to give a one-hour lecture summarizing Lewis Henry Morgan and the modern study of kinship, nor can I imagine how clear the ideas would be to students after such a lecture.
Ultimately, I think that this is a form of the curse of knowledge: as instructors, we might think that the ideas are easy because we are so familiar with them, or perhaps because we never had difficulty with them ourselves, but that doesn't mean that they are actually easy. We have to figure out how to put ourselves back into the shoes of a novice, and remember how difficult the ideas were in the first place.
The impact on morale
Because mathematics is so often stigmatized, I think that mathematicians are (or, at least, should be) particularly sensitive to things which are likely to further drive people away or cause them to shut down. However, I think that all instructors across all fields should be aware of how their style impacts morale, so this isn't really a math-specific observation.
If you tell a student that something is "easy" or "clear" and that student is still confused, you are implicitly suggesting that the student has failed to grasp an obvious concept. This tells the student that if they admit to their confusion, they are admitting to being inferior, and that they might become an object of ridicule. The student understands the failure to be theirs, and theirs alone, rather than a failure on the part of the instructor, or a blameless failure in the process of communication. Such a student is not going to ask a question, is not going to seek clarification, and is likely to withdraw from the topic.
As such, I think that it is imperative that we avoid saying things that trivialize the work that we and our students have to do. It is bad form to tell a student that a concept is clear, or that a problem is easy.
What might we do?
In the question, it is suggested that an instructor who makes a topic look easy might note how easy things have become. I think that the reality is quite different: the teachers who make things look easy are those who are most sensitive to how difficult things really are. They lean into that difficulty, and explain clearly to students where the problems lie. It is then up to students to come out the other end and say to themselves "Wow! I thought that idea was hard, but the professor really made it easy to understand!"
I don't think that there is a one-size-fits-all solution to this, but there are a few things that I do in my own practice which, I think, help:
Ask Questions: I don't assert that ideas are clear; I ask if they are clear: "Is this clear now?", "Do you all understand?", "Do any of you have any clarifying questions you'd like to ask?", "Can we move on, or would you like to work through another example?", and so on. Indeed, I often ask "Is this clear?" in a situation when I know that it should still be quite unclear to students. Done often and early enough, this gives students permission to admit that things are confusing.
Provide Context: To the extent possible, I like to give students some historical context for the development of ideas. For example, we might spend as little as a week defining the derivative of a function. This short period of time belies the enormous intellectual achievement it represents: the ancient Greeks were the first to spot a problem (Zeno's paradoxes), Newton and Leibniz (both of whom "stood on the shoulders of giants") first put the ideas together, Euler (and Fourier and Cauchy) introduced the modern notion of "function", and late 19th/early 20th century mathematicians (Hilbert, among others) nailed down the formalism. If the derivative looks easy, it is only because it took such a long time to refine and distill the ideas. It is actually a remarkably hard concept.
Employ Appropriate Comparatives: It is, I think, entirely reasonable to assert that one example is less difficult than another. Logically, this is the same as asserting that one example is easier than another. However, when one says that a problem is "less difficult" it could still be hard, whereas if one asserts that a problem is "easier" then a student might only hear the "easy". I don't start with "easy" or "simple" examples; I start with "less difficult" or "illuminating" examples.
 Speaking from the perspective of an American, it seems that no one has a problem with a person saying "I'm just not any good at math!" or "I'm not a math person!" Fear and loathing of mathematics is considered appropriate, and mathematicians are seen either as god-like geniuses, or asocial nerds (or both, I suppose). It is considered okay for a "normal" person to be mathematically illiterate. A seemingly equivalent admission of illiteracy ("I'm just not any good at reading!") would be shocking.