Jim, a fictitious character, scored a mediocre 3.48/4.00 in his undergrad (in Physics). It took him 6 years to complete the 4-year program. He is enrolled in a masters program but due to COVID, he missed the on-campus exams. So he has to sit for the exams next year. He is unemployed and feeling very low. Is it possible for him to make it big in academia and get research or teaching opportunities in future?
It is possible but not probable. Also depends on what one means by "making it big." Nobel Prize? I mean, that is a vanishingly small percentage of even top graduates of top schools. Make full professor and have the esteem of some colleagues? More probable, but in today's world, where the tenure track is largely disappearing, still hard even for the most qualified.
The dirty secret of academia is that it creates many multiples more qualified people for the "good" positions than there are "good" positions. The academic labor market is severely misaligned, in terms of ratio of Ph.D.s produced to demanded. So everything else being equal (including qualification and achievement, and those are of course not equal!), most people in academia will not make it big even with the wind at their backs.
Like many areas, academia tends to look at credentials--not only where they are from in terms of prestige, but the manner in which they were acquired (how long it took, and so forth).
In addition, academia also relies very heavily on personal recommendations of advisers, and someone with these credentials (OK but not great GPA, long time to degree completion, missing exams, and so forth) would probably not do well in the evaluations necessary for advancement, like letters of rec, or being thought of when a job comes up that needs filling.
This fictional character would likely need to gain a foothold in some sort of more technical job, like working for a large collaborative project where he could do "grunt" work, demonstrate superior completion of that work, and then use that work up the ladder.