Given the circumstances Alonso should have won Canada (or at least challenge BMW's for a win, he had the pace), but he binned it instead.
Given that Piquet scored 2nd place in Hockenheim, I'm sure that Alonso at his best would probably be better than Piquet at his best and would manage to bring the car to a win. That's 4 possible victories for Alonso in 2008.
You cannot put all the bad stuff on car and all the good stuff on driver, just doesn't work like that.
Piquet fluked into 2nd place in Hockenheim. He was nowhere, but SC played in his favour. Canada 2008 I didn't remember, but after a quick check-up, it was one of the examples of overdriving by Alonso. He could have won, but, not by merits of the car, but due to attrition.
Overall, R28 was quite bad at the beginning of the year, and Alonso overdrove it, causing avoidable mistakes, a bit like last year, only that in 2008 those mistakes were more costly pointwise (and the car was way worse than F10).
In my opinion, Alonso overdrove in the first part of 2007, 2008 and 2010, and that was the source of his problems those years. Well, realistically, 2008 he had no car for anything until the last part of the season, where both the car and his driving improved quite a lot.
In 2007 Hamilton got under his skin and he made silly mistakes in Barcelona and Canada (that was probably his worst race ever). He just could not accept that Hamilton sometimes could be faster than him, and that was his undoing. Ron Dennis, instead of reigning in the quarrell between his two drivers allowed them to put him between a rock and a hard place, and eventually he chose Hamilton. From then on, Alonso's character kicked in and he focused, laser-like, into getting as many points as possible. The rest we know.
Last year it was, IMO, something similar. He overdrove the car at the beginning, and when everything seemed lost, his stubborness kicked in again, and he finished the season on a very high note.
In evaluating whether those three seasons are bad or good, the key factor probably is deciding what should carry more weight: getting into troubles during the first part of the season, or being able to recover from those during the second.