Yes, suppresors can greatly decrease accurcy. My SR-25 from last deployment was a great example of just that. I zeroed and trued the weapon with out the suppresor to insure it was a good weapon and it was holding 1MOA. I put the suppresor on and questioned my own ability to shoot the grouping was so bad. I had two other fellow snipers shoot the weapon with the suppresor and without and we all came to the same end; for that weapon the suppresor being added made the gun completly unuseable. However, same deployment, my M4 was suppresed, and I had no issues even at distance with it.
Any time you intorduce any new variable to the flight path of a round (strong wind, temp change, barometric pressure, altitude, striking glass, passing through a bush, cleaning the weapon differently, anything) the round can be greatly effected.
Hope this helps.