Is it appropriate
Yes it is. Science is about exchange of information. Unless the other researchers are on a rival group that hate you, I see no reason for you not to ask them. Put yourself in their shoes, and you will see it is actually in their own interest to help you. Two things that might go through their head:
- Someone is asking me about my research! I will help them so they can cite me, my h-index will go up and I will be promoted sooner!
- Someone is asking me about my research! I will help them so they will invite me to their next symposium/colloquium/workshop to speak
about the subject!
By asking them a question, you are not making the impression that you do not know. Assuming they help you, you should know afterwards. You are learning, and people also learn by explaining themselves. Maybe your question will make them think of something they didn't before? This starts a discussion and opens up new opportunities. Who knows? You might even collaborate in the future.
This discussion is also beneficial if you meet them later on, let's say in a conference. Instead of saying "hi, I'm convexityftw" followed by an awkward silence, you can start with "hi, I'm convexityftw and we talked before on the subject of...I did this...got that...". You're getting your name known, and this is just one form of networking.
If so, should I mention my advisor at all?
No, in my opinion. Being a PhD student means you're being trained as an independent researcher. It's a matter between you, researcher A, and the author(s), researcher B. Your advisor should not babysit you or give you the answers. It's your job.
asking such a stupid question
- There are no stupid questions.
- I don't know in how many seminars and conferences you've been, but I've heard so many "stupid questions" from senior professors, so I don't worry about it anymore. I'm in good company and we're all asking stupid questions.
I will finish with some anecdotes from my own experience (a last-year PhD student in the earth sciences):
I send these kind of emails all the time. My advisor doesn't even know or care that I do. I never got a negative reply in the form of "I'm busy I don't want to answer you" or similar. I was ignored only three times. I don't even remember how many positive responses I got - several dozens? Some were more useful than others, but positive nonetheless. I met some of these "important" people at later conferences, and we had good and productive discussions. Now they know me as "the guy who knows X and is good with Y". Only good can come out of this.