Traveller and other SF games takes a simplistic view of a great many things. This is a necessary and valid design decision, otherwise most people wouldn't have the technicals background to stay alive. So we have maneuver drives to keep us from figuring out delta vee appropriate missions. We have planets reduced to the simple and concise UWP.
Let's talk a little about atmosphere. First most science oriented people will note most major worlds are about 8,000 km in diameter which is a shade too light to keep any atmosphere you'd not find in a neon lighting tube in the long term. This could mean most planets in a CT style setting are already oddball, ether more massive than their size indicates or having a breathable atmosphere through other means.
Maybe your otherwise Mars-like planet was the target of terraforming by humans or beings with lungs similar to ours. In the case of ancient aliens this might mean relics and other goodies are lying around (maybe the engines of terraforming are still chugging away). In the case of humans ... well terraforming is big bucks. The regime or company doing it needs a reason why they need that planet habitable and are likely to own that planet and its people body and soul until they stop seeing a profit. If the terraforming ended then you might have all or many cast off machinery left behind for scavenging.
Biotech is a cheaper alternative to machinery and might create a sustainable ecology and environment for millennia or aeons. Your Lithophages eat rocks and break them down to oxygen and silicon. The rest of the tailored organisms do whatever they have to do to keep the lithophages happy. Settlers earn quickly not to build houses out of stone. Bear in mind such a planet might have the terraforming organisms in place but not the complex ecology necessary to support humans, many of whom might not be on a Vegan diet and require cattle to be raised ... or Vegans. I don't judge.
Most adventurer types will have two questions about atmospheres. Is it the right gas mix? Does it have contaminants? The more adventurous will grab a compressor and take a stab at breathing the really thin atmospheres.
Unfortunately there is still the matter of pressure. Pressure will kill or cripple you way quicker than most contaminants and more surely. Just ask Alexei Leonov.
Most ships will of course change their internal pressure to match that of their destination. Ships that are crashing or require rescue operations will not have that advantage. Basically going from a thin atmosphere to a dense atmosphere will be very bad for you unless you take a half hour to an hour to recompress. There shouldn't be a problem going to either from a standard atmosphere.
A case in point was the canny merchant who was tired of being stopped by a border patrol for what amounted to extortion and unable to afford weaponry. The merchant pressurized his ship to minimal levels in jump space. When the 'patrol' stopped him for inspection their boarding party came on board and was subjected to the full effect of going from 1 atmosphere to one tenth atmosphere pressure when the over-riden airlock opened. Almost instantly subjected to an excruciating case of the bends they could only watch as the merchant crew seized their weapons, restrained them and then left a demolition charge in the cruiser's airlock for a parting gift before tearing free of the docking clamps and maneuvering to where they'd have a good view of the blast. Moral of the story is don't pick a fight with someone concerned with profits and not the glory of battle.
The bottom line is prep your passengers and crew for their destination en route or you may all spend a couple hours in a decompression chamber. Even a compressor mask doesn't acclimate you to pressure differences. Spacesuits will of course although most of those are designed to be worn at less than an atmosphere of pressure to make them easier to wear and work in. The first Terran spacers spent hours in decompression breathing pure oxygen to get into their primitive spacesuits. Exercise and medication will get that adjustment time down to 15-30 minutes depending on the suit. But long duration suits of any tech level are light for long term wear and thus are pressurized as little as possible. Survival suits can be donned instantly of course and are not the best to work in.
People talk about ships performing wilderness refueling and being crushed by the immense pressure of a gas giant because of a misplaced decimal in a calculation. On the bright side the pressure will kill you long before the ship gets crushed.
Planets with a thin atmosphere are dangerous in a different way. Low pressure makes your body struggle more to take in enough oxygen. Sure the planet's atmosphere is listed as breathable. The locals like the atmosphere. But the rating doesn't take into account exercise and exertions such as combat. Also the natives have been there almost as long as the rocks and have played evolution's game successfully. Your crew might get tired quickly, fall asleep at the wrong time or be penalized in their actions. Altitude sickness is no joke. Many mountain climbers have died from it.
Altitude compounds all these problems. A mountain on a thin atmosphere planet may have a very thin atmosphere at its peak or even lower if the gravity is higher than Earth normal.
Then we have planets with weird atmosphere codes like D. The atmosphere is breathable on mountainous regions but denser and more deadly near the surface. Often only a few kilometers vertically separate the two. The atmosphere of Sofar is one extreme case. At the surface the pressure is a good three atmospheres and extremely humid with a high oxygen content. The few ships that land there use the Pinnacle, a landing pit made by leveling a mountain top (local legend says the colonists detonated a pirate ship's generator to do the trick.) Raiders came to Sofar after hearing about the local mines of precious metals and jewels, landed at the Pinnacle, and descended the three kilometers to the surface mines along the twelve kilometer Trail of Piety.
They used their TL 10 weapons and armor to intimidate the miners into paying a tribute and then retreated quickly up the Trail of Piety, dotted with small shrines. The locals pursued in greater and greater numbers and the raiders decided to high tail it, their morale boosted by the fact that the superstitious locals had to stop at several shrines for prayer.
What the raiders ran into was a case of the bends. The locals praying were taking enough time to decompress. The raiders in their eagerness to escape forgot about the air pressure. After the locals removed the thieves' armor and gear they unceremoniously dropped them over the side of the trail. Moral, don't take any trail littered with human bones.
Oh and watch your pressure readings. They aren't just for scuba divers.