OK, if you want to get technical, a "license" per-say is not needed,
though
they still need to be "certified as technically qualified to perform
transmitter maintenance and repair duties in the private land mobile
services and fixed services by an organization or committee representative
of users in those services." - Reference FCC Part 95, Section 95.424
subsection "b".

And as the question that was asked, in relation to the US:
"Are CB's allowed to be repaired by the average CB'er who has the
knowledge
or are they only repairable by a qualified technician of some kind[?]"
The answer to the first part of the question is still NO, and the second
part is still, YES. The "average CB'er who has the knowledge" can NOT
service their own CB, legally. They would need to be "certified" to do so
"by an organization or committee representative of users in [the CB
service]."


Don't leave out the word "should". It does not say MUST.