Even without a Hostage Crisis, the revolution-induced oil crisis will still keep the economy in terrible shape - so Carter will still be dealing with a difficult economy and without the rally-round-the-flag effect, Kennedy has a much better chance of defeating Carter. Most likely, Carter manages to defeat Kennedy but still loses to Reagan albeit by a smaller margin.
That said, this is an interesting scenario, because without the Hostage Crisis, the entire history of Iran and Iran-US relations will be drastically different.
The Hostage Crisis was a very seminal moment in Iran's history. The atmosphere of crisis allowed the Ayatollah and his followers to consolidate power, purge moderates, and was what actually ended US-Iran diplomatic relations, which both sides were trying to repair at the time.
Without the Hostage Crisis, the moderates are in a stronger position, the Islamists in a weaker one, Iran and the US don't break relations, and Iraq doesn't dare invade Iran. Possibly also meaning no Gulf War, meaning no US bases in Saudi Arabia, no Bin Laden... lots of butterflies from this.
In Iran itself, the clerical regime either falls or is moderated significantly once Khomeini dies. Iran might still be called "Islamic Republic" today, but the Supreme Leader will probably mostly a ceremonial position, and the country won't be closed to the West and will probably be substantially wealthier.