Yes, in that example, the only difference would be the deliberateness, and as a potential difference, the degree of the concern.
気にする is to be concerned about something to the point of being bothered/disturbed by it.
気になる could also mean the same, but not necessarily so. Depending on the context and tone,
気になっていた could be anywhere between lighthearted and grave.
気になる are not always interchangeable.
気にする is to mind/care/worry as a generally negative emotion, while
気になる is broader and can be a neutral or positive feeling as well. So, when you are “interested by” rather than “concerned about” something, it is always
〜なる and never
Here is an example that I think illustrates the different connotations. Let’s say you are talking about an artist who unveiled a new painting:
“He wondered what people would think of his work.”
Similar to the usage of the word “wonder” in English, whether the artist is simply curious in what people would think, or whether he is somewhat worried about their opinion, is not apparent in this one sentence. It is context-dependent.
“He was concerned with what people would think of his work.”
This, on the other hand, shows a value judgement of the effects of the artist’s interest in people’s opinions. You are saying that his interest affects him in a negative way. The extent of these negative effects can range anywhere from “it makes him a tiny bit anxious” to “he is preoccupied by it” to “he is destructively obsessed by it,” but in any case you have made a judgement that the effects on him are neither positive nor neutral.