Absolutely intentional, the pirates were most certainly designed to look cartoonish. Marc Davis' original concept art showed very comical facial features and expressions that aren't remotely realistic, and the figures were designed after his designs. Here are some samples, all the others are similar in nature (even the redhead beauty looks fresh out of a Disney animated movie)-
It had nothing to do with technology limitations, creating more realistic human figures was well within Disney's capability by that time. The Carousel of Progress and Lincoln exhibits at the Worlds Fair both predated POTC by three years. But the human animatronic figures in those two attractions were designed specifically to look more realistic compared to Marc Davis' pirate designs (Lincoln in particular is reported at the time as actually fooling a good number of people into thinking it was an actor instead of an animatronic).
Similarly, the far newer World of Motion ride at EPCOT was also deliberately designed with somewhat comical human animatronic figures similar to those in POTC (I believe Marc Davis also designed the characters in Motion). And it, like POTC, had a more comical side to it. Whereas American Adventure, Spaceship Earth and Horizons (two of which were designed, built and opened at the same time Motion was) all utilized realistic human exterior features.