First, its generally not good to go wide open at low rpm, the carb is sized such that it makes good power at high rpm wide open, at low rpm with wide open throttle the air velocity drops too low and the fuel doesn't atomize all that well. Try opening it a bit more slowly and notice at what point the bike is pulling best in releation to throttle opening (say at 2/3rds throttle) gradually feed in more throttle as rpm's pick up. Secondly, there isn't much reason to ride at low rpm's on a 125cc bike imho, use the clutch and trans and keep the motor singing.
If you feel that there is a problem (other than the operator:)) look into your carb jetting. A low rpm bog is usually from a lean condition at that rpm/throttle opening, but could also be from loading up. These will be two distincly different bogs. If its too lean, opening the throttle quickly will make it worse and there will be very little smoke. If its too rich and loading up it will be blubery, smoke a lot and eventually clean out and take off quickly.
If you determin that jetting isn't going to solve your problem make sure the powervalves are working properly, the reeds are sealing, the top end is in good shape etc.
good luck