This vile group is so filled with hate and barbarism they will do anything to get their point across. As they lose territory and their leaders are strategically killed they have turned to desperate moves of mass killings with any manner available to them. There is a bloodlust from these groups that transcends any degree of evil I have ever seen.
ISIS is losing ground in terms of extending its 'caliphate' as armed forces are taking back control of their lands, so one way of showing some sort of power is to attack soft civilian targets. The easiest way to do this is through suicide bombers. This also creates a desired atmosphere of fear that makes ISIS leaders feel some sort of control but to the outside world it looks like desperation.
Suicide bombings have been a strategy of attack for thousands of years. Therefore this doesn't mean ISIS is becoming desperate. ISIS cares about one thing, causing harm to the human population mostly through terrorist attacks. They don't care how they succeed in this goal as long as they are causing harm.
ISIS and other terrorist groups have always used suicide bombings, so the fact that they still do isn't a sign of desperation. These bombings serve not only the purpose of scaring and hurting the enemy, they are a recruiting tool. People are invited to join ISIS to become martyrs for the cause, and this attracts certain personality types.