The degree to which refrigerant appears bubbly in the sight glass depends on which refrigerant is in the system. For R-12 it is normal to see only a few, if any, bubbles. Indeed, the sight glass exists to aid in properly charging an R-12 system.
If the system is charged with R-134a, then the sight glass is useless. With R-134a the state of charge cannot be determined via the sight glass - there are always bubbles present, even when the sytem is over charged. In later models, MB eliminated the sight glass - my 210 does not have one. In this case a set of refrigerant guages is required to check the high & low side pressures in the system.