Is it problematic to use a Wikipedia article I wrote in my PhD thesis?
No, but make sure you cite it and mention it, to avoid being accused of copying and pasting from Wikipedia.

In any case, it is great that you have contributed to Wikipedia and I wish more people considered using pieces of their introductory chapters for Wikipedia. Just make sure that people know that you copied to, not from, Wikipedia. (I mean, the order is irrelevant, i.e. whether you first wrote text for Wikipedia or your thesis, but the authorship is.)