Traveller is a game where you're not meant to be in a fair fight. And a lot of PC scheming goes into making any fight as one-sided as possible. This is a completely valid, rules-as-written way of playing that's a lot of fun.
But, since you're asking, you don't have to play it that way at all. You can in fact add in some simple houserules to make the PCs pretty heroic. I've done it in the past and it's a different game but no less fun. Here's what I'd do - which is some combination of the stuff below. You might treat it as a list of options rather than something to grab all of them - but again, if you want very heroic play, feel free to grab the lot.
a) The luck stat from the Traveller companion is really good. The PCs end up with a pool of points to avoid any dice roll they want, but they have to spend time earning them back or waiting for the end of the scenario. I think this might help you because you can literally shoot a PC in the head (just once) - and they can spend points to make that miss. It means character death mostly happens the second time they get into a lethal event, rather than immediately.
b) Add bennies. Again, I've done this in some games and I find it to be game changing rather than game-breaking. You simply say that a particular type of action (let's say something that PC would define as heroic - or simply cool roleplaying that emphasises character and everyone likes) - gives a bennie. Which can be say a poker chip. Then in any situation where the PCs fail a dice roll, they pay the bennie and get a reroll. If they fail a second time, take the consequences. I think this might help you because it makes PCs much more competent but still able to fail.
c) Enforce a floor to stats in character-gen. When the players roll for stats, tell them that if their dice don't all add up to more than the average (42 for six stats, or 49 if you have luck as a seventh) - then they can reroll 1's. And if they still don't make it, they can reroll the lot. That automatically removes the weak end of the PC pool. I think this might help you because it makes the PCs less stuck with mathematically weaker characters.
d) In character-gen, make skills less random. I.e. when they are rolling for the skill gained from a profession, let them pick any that match the dice roll (same row) that their character could have got, rather than making them choose the column in advance. I think this might help you because it let's the players be a bit more directed in where they end up building their character. It's a sort of middle-ground between a lifepath and a point build. You'll probably see them pick one skill and have at least 3 in it this way because players like to be good at something. But then more heroic play is all about that.
Anyway, all this will make the game "not Traveller" in some ways. But I don't think it'll be any less fun. And if your group wants something more like D&D, that's how I'd arrange it.