How to cite lecture notes? (and should I even do this?)
You should make a good-faith effort to find and cite original source of the results (to give proper credit).  You should only cite the lecture notes if (1) they are the original source, or (2) the original source is inaccessible, either literally (out of print or unpublished) or figuratively (written in a foreign language, with excessive generality or formality, or just badly).

Finding the original source may require significantly more scholarly diligence on your part than the author of the lecture notes, since most lecture-note authors (including myself) are fairly sloppy with references.  Such is life.