Should I include a DOI/website URL for a publication in dissertation bibliography?
Adding DOIs and URLs to your bib references is really a good idea!

If you are really concerned on the visual clutter, you can "hide" hrefs under the author/title/journal line. In this case, the references are not visible in print version (which is probably not terribly bad, since only a few people will bother to type DOIs from paper back to a browser search line). In the pdf version, however, one can click on the bib reference  line to access the online document (which is really something people appreciate). If you hide hrefs under the titles, make sure your PDF readers do understand this (give them a clue with a footnote or so).

It is really a good idea to use DOIs as much as possible and avoid using potentially unstable URLs. However, if the document is not easy to find (e.g. not among first 3 references in the Google scholar), I would probably go with the URL to help navigate the reader.