It's colloquial language. "Easy to look at" is a joking way of saying "attractive." The idea is that an unattractive person hurts the eyes and is difficult or unpleasant to look at, but an attractive person is easy to look at.
"Not to mention..." introduces an additional thought, a side-issue that isn't part of the main idea.
These three sentences each say the same thing. In the 1980s they would all have been acceptable in the United States. Now, feminists have made us aware that it is not appropriate for men to be constantly telling everyone that they are paying attention to how women look.
1) "She's a very good secretary, plus she isn't hard on the eyes."
2) "She's a very good secretary, not to mention easy to look at."
3) "She's a very good secretary, and as a bonus she's good-looking."