I have not really thought USA and Japan's relations were that bad per say, but a major Japanese official showing respect to the fallen American sailors at the site of a Japanese surprise attack can only help strengthen the relations. There is probably much more work to be done to bring the USA-Japan relations to a great level, but Pearl Harbor being the source of the biggest conflict between USA and Japan, the Prime Minister's actions should help push good relations forward.
I will look at Japan's Prime Minister Shinzo Abe visiting Pearl Harbor in a positive light and say that yes, the visit quite possibly could be an indication of a more positive relationship between the USA and Japan. I can't imagine that Pearl Harbor is the most desired place for anyone from Japan to travel to so this is a sign of open-mindedness.
The United States has had a very rocky relationship with Japan since World War II. The Japanese were proud of the attack on Pearl Harbor when they did it. The United States became very untrusting. After Japan lost the war, the United States instilled military bases in Japan to stop them from gaining too much power.
The visit by the Japanese Prime Minister is excellent to see but it is highly unlikely to be a new relationship between the two nations. It could have been had the 2016 US election had a different result, but Mr Trump is very inward looking and the chances of him making an effort with the Japanese is very slim.