This is a feature request. See this discussion which explains the request: https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_134#Thank_link_on_Recent_Changes_pages
Here's a condensed version of this suggestion from the discussion:
It would be convenient if there was a Thank link on Recent Changes pages, on each line like how they work in page histories. I use RC pages a lot for patrolling changes, and having a Thank link there would reduce steps I take to thank other users.
It has been suggested that a user should actually view a diff before thanking someone for it, but indeed, I can do that from the Recent Changes page. I can hover over the diff to see it. So, it would be nice if I can "thank" from there without additional navigation.
I should clarify that I'm not asking for the 'Thank' link to be in the diff popup, just on each Recent Changes entry like they appear in each History entry.