Titan is roughly ten times more massive than Pluto or Eris, the most massive known Kuiper Belt objects (KBOs). (Titan is in fact more massive than any other moon in the Solar System except Ganymede.) It would be a rather strange coincidence if the object that was by far the most massive of the KBOs was in orbit around Saturn, well interior to the Kuiper Belt.
Titan's orbit is prograde (it orbits in the same direction that Saturn rotates) and barely tilted with respect to Saturn's equator (less than half a degree), the rings, and most of the other moons. That strongly suggests it formed out of an accretion disk around Saturn, as the Galilean moons formed around Jupiter.
Titan also has an extremely dense atmosphere, unlike any of the KBOs.
So it's rather unlikely that Titan is a captured KBO.
(Neptun's moon Triton, by the way, is thought to be a captured KBO. But it is only slightly more massive than Eris or Pluto, and it has a peculiar retrograde orbit that is very difficult to explain if it formed around Neptune, but easier to explain if it was captured. And since Neptune is the furthest out of the (known) major planets, it's not that surprising that it could have captured a KBO.)