Is it unprofessional to use graphics from Wikipedia in a poster? Is it an acceptable practice to use simple graphics found on Wikipedia (and hence in Wikimedia Commons) in a conference poster? 

From the legal point of view, this seems to be fine, as long as the image is properly attributed. Presumably it should suffice to mention the source in acknowledgements section. Or am I missing something?

How would this be perceived? Would it come across as unprofessional? The poster I am preparing is for an interdisciplinary and not too formal event, and the graphic I'd like to add is meant to illustrate a basic mathematical notion. (However, more general answers are very welcome.)
The primary issues with using Wikipedia for academic research are that it's a tertiary source, and there's no credibility/quality assurance.

So, you should make sure that


If the image contains intellectual content that requires citation, you should cite a primary source for that content. 
The image (including its factual/intellectual content) meets academic standards of quality and accuracy.


Assuming these are satisfied, reusing images from Wikimedia (or a similar source) is not inherently unprofessional. Of course, if the image is of poor quality or doesn't fit in well with your poster, it will look unprofessional - but this would also be true if you had created the image yourself.

This is, of course, assuming that the image you are using is in Wikimedia commons (not all images on Wikipedia are), and


you follow the license requirements (protects against legal/copyright problems)
you correctly attribute the source (protects against ethical/plagiarism concerns)