Is Intelligence useless for Clerics in Disgaea: Hour of Darkness?

That's not entirely correct.

In the original Disgaea, INT is still the determining factor for the healing amount. In all subsequent Disgaea games, it was switched over to RES. In the first, I believe the RES of the recipient also plays a factor into how much they are healed, but the int of the caster is the biggest factor.

Also, there is no reason for your cleric to be a dual mode (in fact, they become more handy if you learn the boost spells from a mage as well as some damage spells). Just have the cleric make a few students and learn the spells from them by standing next to the students (which gives them access to the spell).