Airing another episode with comedic overtones may have been a minor mistake, since it seems a bit odd to place them together. But this is "Supernatural", and even if the timing was less than ideal, the strengths of the episode just manage to outweigh the weaknesses.
This season (and this series, for that matter) has been focused strongly on Dean and his issues, and this episode is no exception. Dean has serious concerns about Sam and what he might become, and as his fears rise with "ghost sickness", the nightmares bleed over into his reality. There's even a chance to connect some of those fears to his knowledge that Lilith wants nothing more than for him to return to Hell and stay out of her way.
That makes absolute sense for the character, and it pays off all the hilarious moments that came before. Dean is always such a tough customer that his fearful reaction to anything and everything is beyond bizarre. Yet, in a nice touch, it's never quite over the top. It comes very close, but the line is never crossed. That's important, because if it felt like something that couldn't happen within the "Supernatural" universe, then it wouldn't have worked. The best self-parody takes place within continuity.
Despite my enjoyment of the episode, it was easily the least effective of the season thus far. The previous episode may have been absurd on several levels, but everything was designed to fit into the question of "what makes a monster". It complemented the episode that come before ("Metamorphosis"). This episode seemed to be designed for a dual purpose: to touch on Dean's deeper fears about Sam, and to allow Jensen to show off his comedy chops.
The exploration of DeanÃ¢â¬â¢s fears took very little time; the lion's share of the material for Dean was played for laughs. I personally found it all very funny, especially the bonus footage at the end of the episode. But those who found the previous episode to be little more than semi-inspired filler (or just garbage) will no doubt feel the same way about this episode. And given the strengths of the deeper, darker elements of the narrative this season, two light-hearted episodes in a row are a test of audience patience.
That said, how many shows would kill to have an episode like this as the low point of their season? A lot of shows struggled out of the gate this season; "Supernatural" has been topping the curve since the premiere. With a Halloween-themed episode coming up next in the queue, the writers need to be very careful and avoid pumping out three non-standard episodes. The survival of the CW network is no sure thing, after all, and there are more important and better plot threads to explore.