Like the title says, I'm looking for the theorem or the effect that makes you feel incompetent compared to a master (let's take playing music for example) but good enough that others that aren't in said hobby find it impressive.
I thought of the Dunning-Kruger effect but it's not exactly that.
In the land of the blind, the one-eyed man is king. Per Wiktionary:
Even someone with limited abilities or opportunities is dominant over, and considered special by, those who have even fewer abilities and opportunities; the value of any ability depends on its prevalence.
You are a big fish in a small pond.
Being a big fish in a small pond refers to a setting where you are more powerful/important/influential than your, rather small, peer group. [A person whose expertise/ability is probably overestimated by the laypeople around them.]
Two more options:
Impressive to the untrained eye is rather an idiom than a full expression, but seems to capture the essence of the intent quite well. Although it seems to get mostly for the opposite, namely when something looks unimpressive to the uninitiated.
And, in case you want to highlight that despite there being quite some road ahead, there has been progress, one could go with a beginner among masters, but a master among beginners. Not that good a fit to the original request, but related enough to my ears to include it here.
You know enough to be dangerous.
A little knowledge is a dangerous thing.
When somebody has basic competency in doing something, but is not an expert, so he or she can potentially make mistakes that might break something.
Impostor syndrome seems close. However, it suggests the person experiencing it is more skilled than they believe, and are being overcritical of themselves.
From wikipedia:
Impostor syndrome, also known as impostor phenomenon or impostorism, is a psychological experience of intellectual and professional fraudulence.[1] One source defines it as "the subjective experience of perceived self-doubt in one's abilities and accomplishments compared with others, despite evidence to suggest the contrary"
In that situation, I'd probably refer to myself as a journeyman. As in "journeyman carpenter" or "journeyman mouth harpist".
Its got a traditional technical meaning, as tradesman who has completed their apprenticeship, but is not at the level of a master. However, that kind of formal trade apprenticeship/guild arrangement is uncommon now, so its more often just used to describe someone who can do the work of a craft acceptably, but is not at the level of a master craftsman.
Since you used the term "master" in the question itself, and basically described the relative level of a journeyman, IMHO this is The Answer.
I'm not sure there is a term for exactly what you describe. I think the problem is that such a term would only be utilized by someone with a third person omniscient point of view. That precludes everything short of fiction and subjective essays.
Imposter syndrome alone is an relative description--you feel incompetent compared to another. And while it may describe how you feel about yourself, it does not describe how others feel about you. You could very well be a master, but not feel it. It doesn't speak to the fact your audience is impressed.
There is a term called competency porn which describes the phenomenon of taking pleasure as you watch someone who is good at a task, do well at that task. It doesn't capture the feeling of impostership, though.
The POVs of both you and the audience would need to be known and synthesized by a third party in order to have cause to then accurately describe the situation.
That happened to me quite a bit when I was in college. I called it "involuntary oneupsmanship." Thinking of that British comedy (with Alistair Sim?) that popularized the idea of oneupsmanship.
