First off I've never been an Earnhardt fan (Sr or Jr) but it seems that Jr. is getting jumped on because he is popular...not over-rated.
As for his driving ability he is not great, may never be, but he is not void of talent. To me a Top 10-12 driver in the Cup Series is pretty darn good no matter who you are or how you got there.
No other driver at DEI had even come close to the numbers he put up there, and to be honest I never thought that DEI was comparable to Hendrick or Gibbs but Jr. was competitive with those teams.
Also, just going on 2008 numbers, Jr's first year with Hendrick was just about a carbon copy of Kyle Busch's 3 years there. Kyle averaged(per year) 1.3 wins, 10 Top 5's and 17 Top 10's. In 2008 Jr. had 1 win, 10 Top 5's and 16 Top 10's.
The thing with Jr. right now seems to be communication and his ability to relay information to his crew as to how to make the car better. Whatever the problem is it has been magnified this year. If that problem can be improved I think you will see that team improve, but that maybe a big IF.
As for being popular, Dale Jr. was like the heir to the throne, the crowned prince if you will, of NASCAR. Also since he did show some ability and had a great personality, the allegence his father had of course transfered to him.
His family name is synonymous with this sport and their following is huge so of course he would be the "poster boy" for NASCAR and a host of other things because that is what sells(love it or hate it), and the Earnhardt nation is fiercely loyal as is most of the die-hard NASCAR faithful.
Even though he may not be the best out there, I have no problem with his popularity, he adds a lot to this sport for more than just his name. If you don't believe me go to a race and see what the fans are wearing, nearly 1 out of every 2 will have 88 stuff on. A good many of those I'm sure don't just back him because of his name, its because he's a likable guy who can drive a race car. You take away Jr. you'll have hundreds, if not thousands more empty seats each week.