I was surfing SO on my iPhone, and wanted to see the info (tag-wiki) page for a tag, but didn't find it. Clicking on the tag always leads to the list of question, and there is no other link to the tag wiki. Is it not available on the mobile site?
Yes. It looks like you are correct. There is simply no mobile version of tag-wiki pages. Arguably there would be no need to link to a page with no mobile version from the mobile version.
I would be OK with that argument if it was consistent behavior, but there are links to many non mobile compatible views from the mobile version. Revision histories is the first thing that comes to mind. Also your helpful flags are not easily accessible on the mobile version.
Excerpt from the request -
One important thing I feel is missing from the mobile design is the ability to view a tag's excerpt directly on the tag page. I find the excerpts very useful on the main site when I see a tag on a question, but don't exactly know what it's about.
No, they evidently are not. You can, however, get to the tag wiki page by manually navigating to
/tags/tag-name/info. Note that that will give you the full site version.