im surprised that no one made the point im about to make yet.
the answer is it depends on why the person is wearing jeans. if the person, fully cognizant of our communities standards of dress, decides to start wearing jeans in direct contradiction of that standard then yes, it indicates a problem.
if the person starts wearing jeans because he legitimately finds them more comfortable than perhaps its “mumar l’teavon” or perhaps its not mumar at all and he just likes being comfortable. for example, i stopped wearing a hat during the week about a year after my bar mitzvah because it was very uncomfortable for me and no other reason. i didnt feel any sort of rebellion in it and it was no indication of my frumkeit.
if a guy starts wearing jeans because his job has no place for conservative elegance then there’s nothing to talk about. even if his job doesnt involve manual labour, but it doesnt necessarily call for a dress shirt and slacks then i dont see a problem. it also depends on the age of this hypothetical fellow. judging a person based on the fabric from which his pants are made is risky business and should be done strictly on a case by case basis.