This is actually a rather interesting question, and one that does show up in many depictions of infantry combat, and even just ground and shore based formations in the Pacific, Thin Red Line, Flags of My Father, Ba Ba Black Sheep, etc all depict it to some extent.

Though before we get into it:

Decided to start watching the Pacific recently. If you don't know, it is basically Band of Brothers but with Marines in the Pacific ocean rather than paratroopers in Europe. It is not as great as Band of Brothers, but what is?

Piss off with that nonsense. At the very least Sledge and Leckie weren't serial plagiarists who fought extensive battles with the truth.

But with that out of the way, the answer is basically yes for a whole lot of reasons uniform standards quickly slipped during the island and jungle fighting. Between issues with supply and providing sufficient changes of clothes, vs prioritizing ammo, food, water, and medical supplies from ship to shore. The at times inhuman humidity or blazing sun on the various coral atolls and jungle islands.

And as units very often spent the entire battle on the island in question if it was too small, or weeks to months before being rotated out if larger there often was an attitude of if it isn't immediately important for fighting efficiency it can wait till after the fighting.

But that only applies for the sharp end of the spear, what about in supporting locations, assembly points for the great amphibious armada, or say air bases supporting forward locations.

And the answer is basically that yes in many places, for much of the time, the officers and men would work in or wear what they could be comfortable in. This was not something hard and fast, and absolutely could be a function of predilections of a units officers and NCO's. But abundant evidence, and anecdotes exist for it being the way of things in places across the Pacific.

For some good photographic depictions of a major installation that was not immediately on the front lines I would point you here to get a good first impression: https://imgur.com/a/mOvzk

It was assembled by a user on r/warshipporn and depicts the history of Ulithi Atoll. Which for 6 months or so in 1944 was the primary forward fleet base for the Fast Carrier Force and the great Amphibious and Logistic train that followed. It also served as an airbase for Navy and Marine Aviators, and a recuperation nd rest point for men traveling through and had extensive repair facilities along with a large fleet anchorage.

We see Sailors and Marines at work and rest in multiple states of undress repeatedly. Even when it seems ill advised like when working on a Corsair's engine. But we have other times such as when taking a break and looking out at the fleet anchorage. While officers were a bit more according to regs, even they were not impossible to find stripped for comfort.. Not the dude in the end in his undershirt, while the sailor behind the bar is too.

It also of course could be very contextual, down at the beach or on the baseball diamond that was set up of course virtually everyone was stripped down. While when an Admiral was visiting suddenly everything was nice and tidy.