It is a big win for the Democrats. Now Kentucky will have a father and son, Steve and Andey Beshear as governors. His father was governor of Kentucky prior to Bevin. I do think this has to be put into perspective though. Bevin was the worst liked governor in these here United States.
Matt Bevin, Most Unpopular Governor in the Country, Stakes His Re-Election on Trump
It also needs to be pointed out that the Republicans won every other statewide race. Whereas Trump had a 53% approval rating in Kentucky, Bevin had a 33% I'm not surprised, after all Kentucky elected his father governor. Personally, I think the folks of Kentucky regardless of political stripes were just plain fed up with Bevin. He needed to go and gone he is. As you pointed out, Trump won the state by 30 points, that shows you how unpopular Bevin was. In fact Bevin won the Republican vote 81-16 while Beshear won Democrats 94-6 and independents 59-31. Independents put Beshear into the Governor's chair.
It's also interesting to note that McConnell is viewed favorably by 46% of Kentuckians vs 54% unfavorably. With the right candidate I think McConnell can be had. Although a word of caution, Beven had only a 33% favorable, McConnell's is 13 points higher.
ANALYSIS: Exit Poll Provides A Closer Look at Bevin’s Loss