Not really, cup competitions can be a lottery at the best of times. Some truly great sides have failed to win the CL, while some merely average sides have gone on to win it. That's not to say I'm belittling the achievement of getting to those 2 finals, but to use them as a measurement of how good a side he is building (and in turn how good of a job he is doing) is just foolish.
The league format is the only true measurement of a sides quality, because there's far less reliance on the 'anyone could beat anyone on any given day' factor. Just like under every other manager over the past few years, Liverpool have been miles away from winning the league under Benitez. There's no way he would still be Liverpool manager without those 2 CL finals, so it's fair to say the support he gets is based almost entirely on them, beacuse he's done **** all in the league to make the fans so loyal to him.
At least the support that Wenger recieved when Arsenal were off the pace was based on a proven track record of building great, winning sides.