Does mana cost matter at all for Witch Doctor skills?

Once you start getting into the stronger spells, you will likely run into mana issues unless you have some sort of strategy for boosting mana. Otherwise, you'll have to pace yourself.

Acid cloud and Locust Swarm are two spells I use fairly frequently. If I open with Grasp of the Dead, then cast two Acid Clouds, I won't have enough mana for Locust Swarm.

It really depends on your build, though. Many pet-centric builds may not really require much in the way of mana assistance, since minions are "fire and forget" abilities.

As with other characters, most of the Mouse 1 Active skills are in effect infinitely castable (except Plague of Toads), as the mana cost is moot due to regeneration. Beyond those, however, the other spells will significantly dent your Mana pool, especially if you are trying to resummon pets in battle or use channeled spells (again, just like other classes who consume a resource they generate with a primary skill, Spirit, Fury, Hatred)

An example of the latter is the first you might come across; Firebats will crater your mana.

At your level, you should already have firebats, which will definitely drain your mana pretty quickly if you use it continuously.

But it really depends on your build. Skills like poison dart and spiders are not heavy on mana use, and you probably won't have to worry about it. Later skills will definitely be constrained from quick use by mana regen rate.