An amateur is an enthusiast, a lover of the thing. You can actually see that in the root of the word, ama- (love).¹ Amateur is often used in opposition to professional because amateurs frequently work for the love of something, rather than for pay. There is nothing about amateur which automatically implies less expertise than a professional (academic or otherwise) might be expected to have. In context it might have that connotation, or might not. These two senses of the word can be found in the Oxford definition of the term:
1 a person who takes part in a sport or other activity for enjoyment, not as a job
The tournament is open to both amateurs and professionals.
2 (usually disapproving) a person who is not skilled
However the closely related adjective amateurish does have that connotation.
not done or made well or with skill
Detectives described the burglary as “crude and amateurish.”
- amateur (n.) 1784, "one who has a taste for (something)," from Fr. amateur "lover of," from L. amatorem (nom. amator) "lover," agent noun from amatus, pp. of amare "to love" (see Amy). Meaning "dabbler" (as opposed to professional) is from 1786. As an adjective, by 1838.
—Online Etymology Dictionary