Have the seasons always been?
Has the Earth had it's wobble that causes the seasonal variation in solar energy in the northern and southern hemispheres for it's entire history? 


First it depends on how you define the "entire history" of the earth. There was a pre-earth that was hit by a Mars sized body. The collision fragments ultimately collapsed into the present earth and moon. Exactly how massive the pre-earth was and its orbital parameters are "unknown", meaning we know the parameters with much less precision than we know the parameters today.    

Second there are two factors here. The seasonal (yearly variation) is caused mostly by the earth's axial tilt even though the distance between the earth and sun varies a bit since the earth's orbit is an ellipse rather than a circle. However the wobble of the earth is better better described as the precession of the earth's axis and it currently has a period of about 26,000 years. 


  Is this variation evident in the geologic record or is it an open question? 


There is evidence in geological record for season and precession changes for the present earth. 

Isotopic composition of glacial ice and ocean sediments reflect yearly temperature variations. The yearly temperature changes also cause seasonal changes in the distribution of the earth's weight which effect the precession and tilt. 

Lava cools leaving imprints of the instantaneous magnetic field of the earth over geological time. The interaction of the magnetic field drift and reversal, the precession of the earth, the axial tilt of the earth, and the plate movement of the earth is not well understood. It is basically governed by chaotic system interactions. For example plate movement can cause earthquakes which are as of yet unpredictable. 

Again what the precession and tilt were like for the "pre-earth" is mostly a mystery. 


  Could a meteor impact like that which killed the dinosaurs 66 million years ago have altered or created the wobble? 


The impact that created the moon was with a Mars sized body and definitely effect the earth's precession and tilt.   


  Are there any factors dampening or increasing the seasonal wobble over time?


Moon damps the tilt over geologic time. The instantaneous flow of the liquid magma inside the earth may increase or decrease the precession and/or tilt, but as the earth cools the overall effect is to decrease both.