Celebrities have just as much right to get involved in politics as anyone else. In many cases their celebrity status than help sway the tide one way or another, which is why getting them involved can be helpful. There are also many examples of celebrities who become diplomats in spite of their celebrity status.
Although many people might find it distasteful that people who have no background in diplomacy, no elected authority or, in many cases, little expertise in any practical field, have a say in geopolitical decisions it can not be ignored that, in the eyes of many, celebrities do have a role in using their influence to effect politics.
No, celebrities should not get involved in international politics. Celebrities are generally not trained in negotiations with foreign leaders. There have also been many instances when they have done more harm than good, such as when Dennis Rodman went to North Korea and met with Kim Jong Un and others.
Celebrities should not get involved in international politics. Most celebrities have no more foreign policy knowledge than the average American. The example of Muhammad Ali meeting with Saddam Hussein to secure the release of American hostages was a rare exception, not the norm. The vast majority of celebrities would not be able to have the same outcome meeting with a world leader.