tl;dr -- I have spent the last nearly-fifteen years at a large-ish (5k-10k employees) research-oriented US government contractor and generally yes, I do feel free to express myself where I work (it is a large part of why I have stayed so long!) I feel that I align pretty well with the overarching culture of the organization, and that I have been able to find projects and teams within the organization with whom I have a good cultural fit. This is tremendously important to me, and I am a little surprised at the number of responses that have answered "no"--thank you all for sharing; it has been eye-opening!

Some examples:

I have a poster of a dragon playing a text adventure in my office. I have a shoji screen (that I brought in) separating my desk from my officemate's desk to help me get a little more privacy. I have a few more framed prints of some iconic videogame scenes that I have not yet brought in because the pandemic hit before I had gotten them framed, but I have no doubt that they would be allowed to stand and that some of my colleagues would comment favorably on them. Management supported me in getting a chalkboard instead of the standard issue whiteboard, and in moving it when I switched to a new group.

Upper management has expressly stated support for people wearing what they are comfortable with to work so long as it does not interfere with their work or with safety protocols. (Specifically in response to a question about shorts and t-shirts, so I am sure there are limits to this, but they are well beyond what I am generally inclined to wear).

I generally use Emacs though most of the teams that I've been on have used Eclipse, IntelliJ, Visual Studio, or, more recently, Visual Studio Code; nobody has ever told me I need to change to something else.

I'm a bit of a joker and I find that humor can be an excellent way to alleviate the stress of working under a tight deadline or the frustration of a bug that just won't quit, but I am also aware of the harm that joking around can cause and try to take that into consideration when choosing whether to attempt a humorous statement. I also try to be aware of the culture of the teams that I am on (I have worked on several projects throughout my time here, many at the same time) and adjust my levity accordingly. My colleagues seem to generally appreciate my wry sense of humor and application of sarcasm.

As I get to know a team, and they get to know me and my productivity and contributions, I will sometimes bring a fairly small handcraft to meetings to help keep me focused (especially for longer meetings and/or meetings for which my portion of the project is not the main discussion). Chainmail, knitting, and cross-stitch have all been things I've brought, and while these often draw comment it is generally neutral-to-favorable; I have never been asked not to bring these sorts of things. My teammates also seem to enjoy receiving the trinkets that I was working on as I finished them.

I do not generally get into detailed discussions of politics, and I would say that generally it does feel that the organizational does somewhat discourage those in general (not a particular party or ideology, but overall discussion of politics). That is not to say there is not the occasional grumbling about the latest continuing resolution or a government shutdown disrupting funding, but generally people leave their soapboxes at home.