I think that both those are actually great examples of where using a word like "rational" obscures rather than clarifies.

What about saying "it is rational to feel X"

We have less direct control over our feelings than our explicit thoughts. It may or may not be irrational to endorse a feeling, but I for one think of feelings as neither rational nor irrational, but mere facts, and I think that's the common usage. You'd likely be better off thinking, "Y is true, but feeling X is inconsistent with my goals if Y is true. Therefore I want to feel less X."

For example, let's say that I am afraid of the dark, but it does not in fact achieve my goals to avoid the dark, or to be afraid when it's dark. Then what I want to do is notice that the feeling is unhelpful, and take actions to reduce it.

What does it add to say that the feeling itself is irrational?

What about the actions of someone else? "It is rational to not eat the cake" vs. "don't eat the cake"?

Here, "rational" is substituting for a claim about whether eating the cake has some specific effect, or optimizes or fails to optimize some goal or utility function. It is more precise to make the claim explicit than to use a vague term like "rational." For example, you might say that it is unfair for them to eat the cake, or that they probably wouldn't be better off with the extra calories, etc.