I'm not certain but I doubt an impedance mismatch would cause clipping - more likely to result in a loss of high end and a "dull" sound I suspect. Is this problem a new one or has the pedal always done this? Also is it an old pedal? If it's an old pedal that has changed over time it is possible that the problem is a component that has drifted over time. Electrolytic capacitors are pretty infamous for drying out and drifting over time, but then again there probably aren't any of these in the audio path.
If you suspect an impedance problem, try shifting the pedal around in your effects chain. Some older devices (fuzz faces are the classic example) like to be first in the chain so they "see" the high impedance from the guitar. Try the pedal first in the chain ("sees" high impedance) and after a modern pedal containing a buffer (so it "sees" a low impedance). All commercial pedals that aren't true-bypass - like Boss, Ibanez etc, will contain a buffer.
You can also check out the forum at http://www.diystompboxes.com
and search for information on this pedal. If this is a common problem it will of come up here. At the very least I suspect the guys there would be able to suggest the possible bad components you need to check.