Basically, if defog is used on an enemy Pokemon, would thunder'so accuracy increase?
Not exactly. Accuracy and Evasion are two different stats in addition to the normal 6. The user's moves would have a better chance of hitting that one target due to an evasion drop, but accuracy would still not be raised.
To see the stat multipliers for both accuracy and evasion, look at this picture:
Hope I've helped!
Yes I guess. So lets say ur magikarp uses defog on an arceus. the arceus's evasiveness would be 75% of normal evasiveness so yes, thunder would have a better chance of hitting