It might be fairly easy for ruthless, skilled, and supremely self-confident person to hijack a smaller ship with a small crew and few passengers. The better question is how many such individuals exist and if they couldn't use that combination of traits to engage in some activity that couldn't make them more money for less risk.
For instance if we assume: 1) Jump Drive (one week in Jump Space). 2) Small Ship (six crew or less). 3) Not a particularly high incidence of hijackings in the area (chances are the single person will find an area which is very safe and boring - I'd imagine almost every subsector has a few of these routes). 4) A "living" Traveller universe (as opposed to a RPG one); a universe where the basis is that it is an "ordinary" universe with people going about their lives and the players are the ones who are doing extraordinary things, as opposed to an RPG universe where people are supremely on-guard, things are made difficult to make it challenging for players, etc.
Within that framework it might be fairly easy for one person to hijack the ship; from what I've read of people using modern merchant ships for cruises, extrapolated into the Traveller universe, that the entire crew (and passengers) sit down to eat meals together seems pretty likely on vessels with less than six crewmembers; crews are small, there's a lot of automation, the social difference between 'officers' and 'crew' and 'captain' may be very little or non-existent, there's no "watches" during the one-week in Jumpspace that are so critical that a crewmember couldn't sit down to eat with the other crew. In that situation (and it's likely that this single-person hijacker has researched this stuff before, in fact might have even taken a few passages on the ship before to become even more familiar and trusted), a single person with a pistol might be able to seize a ship (wait until the last day before emerging from Jumpspace, create a plausible story that if the crew cooperates they'll live and herd them into a room then kill them all, etc.); with a better weapon it might not even require that. A single person might even have a better chance of success because the crew is less likely to believe that a single person is going to try and hijack a ship, and thus let their guard down even more.
Similarly, you might even get into the "Hannibal Lecter" realm - in a universe with a low incidence of shipjacking overall, internal security is likely to be pretty lax (or basically even non-existent), crew become increasingly likely to circumvent obtrusive security measures as they interfere with convenience (the more obtrusive the more likely), the single person might simply go around the ship and kill everyone as the opportunity arises (small crew so if the killer acts swiftly, there's a good chance they might do it before anyone realizes what's going on).
Again, with Jumpspace, there's apparently little in the way of external threats and quite little to do during that week - if that week is as "downtime" as a lot of official material seems to suggest, it's likely that a small ship may not even have standing watches - the entire crew might simply turn in at the same to sleep, wake up around the same time (yes, there's maintenance activities and so on which have to be done, but these are always easier with other crewmembers around) on some ship-time. Even if there are standing watches (perhaps to monitor the reactors or something...?) it might even be easier for this single person to convince the watch person to let him/her into engineering or wherever so they can chit-chat to while away the boring hours, play cards, etc.
I know all of these ideas sound like security is criminally lax (and it is), but in a "routine spaceflight" universe of Traveller (I'm contrasting this with the "shipboard encounters" tables in rulebooks which go by "RPG universe" which is going to be more dramatic/interesting than a "routine spaceflight" universe), I have to imagine that "most" crews of the stereotypical J-1 trading vessel that plies a route between maybe 3-5 worlds on a Main sign on as apprentices, gain seniority, grow old, and retire after decades without ever being the target of a hijacking attempt, nor do they know anyone who was a target of one. Of course they hear about them in starport tales as a 'friend of a friend' stories as well from "Imperial Merchant Marine Safety Board" announcements; they probably watch tridees of hijackings with the other crewmembers over beers and laugh ("oh man, who'd sign on to be a crewmember if space travel were really that dangerous as these shows make it out be! When was the last time you heard about a fusion core failure or a hijacking?!") I should think on a safe route like that, crews would get fairly complacent.
OTOH, if you use the shipboard encounters tables and so on as indicative of Traveller's actual universe (and not just to spice up player lives), then things are going to be a lot more difficult. Hijacking attempts or piracy are a not-insignificant events on those kinds of tables. At a 1% chance of them happening per trip, assuming a ship Jumps every two weeks (so about 25 Jumps a year) that means that within four years (once a service term), something like that is going to happen, in which case I think ship crews are going to be a lot more paranoid and take anti-hijacking measures more seriously. (Of course at this point, I have to wonder if any ship captain would consider the income of carrying passengers worth the potential risks.)