Rare, valuable confirmation of Jon's profession! and...
Daniel Clowes once compared the compliment of being a generation's most famous underground cartoonist to being "the world's most famous badminton player." Jim Davis comes right out and depicts cartoonists as universally despised. The gag is no more explicit than "everyone hates cartoonists," so one can only speculate on the precise reasons for Liz's folks' panic. Given the personal problems plaguing a vast percentage of comics artists, perhaps those fears are not unfounded. Not without historical precedent are: O.C.D., chronic depression, impotence, sex mania, alcoholism, BDSM, agoraphobia, womanizing, domestic violence, megalomania, schizophrenia, L.S.D. damage, antisemitism, Libertarianism, Objectivism, perpetual misery, religious zealotry, intense assholism, insanity. More of these lives have ended in suicide, self-destruction, and sorrow than seems statistically reasonable.
The other roles in our world under harsh criticism today are children and parents, which constitutes the entire population. Liz's parents are unable/ unwilling to conceal their disappointment in their daughter's lifestyle choices. Liz, being a strong-willed professional woman of cool, detached demeanor, may or may not care that her parents have expectations of Liz that differ from her own. The Wilsons' disapproval takes the form of a (feigned?) threat to their physical health, forcing Liz to express concern even if she has seen past her boyfriend's social caste and her parents' prejudice.
The parent who expects more of their child than general good health, ability to function in adult society, and the pursuit of personal happiness is setting everyone up for a Catch-22 of doom. The inevitably imperfect offspring can never feel adequate and the tyrant parent will never be satisfied. This cycle begins at birth and does not end until the family tree is burnt to the ground.
One strategy for potential liberation from this loop is through acts of rebellion. Not without its own associated damages, this kind of resistance, conscious or unconscious, still binds one to the wheel: decades into adulthood you're still just acting out against your parents. The more they don't want you to date a cartoonist, the more you may feel compelled to date a cartoonist.
Compare and contrast to Liz's BF having this conversation with his mother. So long has Jon been, well, Jon, that the smallest measure of triumph in his life causes her mind to snap and body to shut down. Note the parallel, though: Jon and Liz's announcements both cause physical reactions and near-suffocation in their parents. Whether overjoyed or displeased, we'll be the death of them.
Also, hmm, Betty Wilson... Betty Wilson... ah, Betty Wilson.