I met a faculty (from a university in the UK) at a conference. She liked and praised my work and we are in touch after conference. She has an impressive profile and her research work is similar to mine. Is it okay to take a recommendation letter from her to seek postdoctoral position overseas? Will it help?
It’s best for people applying for postdocs to have at least one letter from a well-known researcher in the field who is not at their school. It is quite typical that this person has only seen you talk once or twice and has read your work. Of course your advisor’s letter will have more detail and first-hand experience, but an outside perspective on the value of your research is a valuable addition. You should definitely ask for a letter from this professor!
There's nothing stopping you asking for one. However, you need to consider what would be written in such a letter.
In short, you can ask for a letter, but be careful how you use it. What could the person say on your behalf if a potential employer contacts them to get more information about their assessment of you?
For instance, it doesn't seem that you have a professional relationship - you haven't co-authored papers, nor have you worked together in the same institution, nor have you served on the same committees, etc. Any of these things is not necessary, of course, but these are the sort of things a referee would cite as evidence that they are in a position to recommend you or your work.
However, it sounds like your contact does know of your work. However, are they a world-leader in your field? Do they have the respect of your community? When they say good things about you or anyone else, do others pay attention?
How will a reader interpret a reference letter that is based only on a familiarity of your published work, and a single meeting at a conference?
You can definitely ask her for a letter of recommendation. It cannot hurt I assume.
However, considering that the person does not know you much, the letter might not have much weight in the eyes of whoever reads it.
Since there is not much to lose, ask for the letter. If you receive it, use it as you see fit. There is no guarantee whether it will help or will be useless.
Most reference requests come, implicitly or explicitly, with the expectation that the referee will describe in what capacity and for how long she/he has known the candidate. As a result, references from people you’ve met recently or only have passing acquaintance usually do not carry much weight.
Ask yourself how you would start a reference letter if the situation were reversed.
Dear Selection Committee, I write supporting the application of [candidate]. We met once at [this conference] a few weeks ago and exchanged emails afterwards. He seems quite reasonable and full of good ideas but I’ve never seen him in a lab, I have no idea of his academic performance, cannot comment on how well he works in a team etc.
and how you would react receiving such a letter.
It might make a difference in a post-doc application if the position is offered by a colleague of that person, so that this person can supply a final supporting opinion to other strong reference letters.
The situation would become different if you were to have conversations with this person over an extended period of several months, were invited to her lab for some sort of visit or internship, started a project or co-applied for some grant, and generally develop your initial meeting into something much more substantive.
All the answers here assumed that this recommendation letter is for graduate school and if someone doesn't know you very well, its weight maybe is less than someone that know you very well such as your adviser. But, I want to look at this situation from other views, which is not very familiar for all academic people. I believe if that professor will be convinced to sign off on a recommendation letter for you, it means you have a really good scientific reputation that resulted to that someone as an independent researcher endorse your scientific work in a recommendation letter and I think it has even more weight than a recommendation letter from a dependent person (e.g. your advisor). But be cautious cause this is not the generally accepted view in routine academia specially for applying to graduate school but it may have some advantages for other purposes. This other purposes could be convincing someone outside of academia to understand your research and be convinced that it has substantial merit. But again it depends pretty much on your special purpose from getting this recommendation letter, which is not specified explicitly in your question.