The darker the material, the more tempting it is to throw in humor as a leavener. The problem with such an approach is it can turn against you as easily as it's deployed. Instead of the laughs being there to soften you up for the next gutpunch, they're more like the show's way of nudging you in the ribs and saying "Oh, never mind, I was only joking."
Sankarea: Undying Love is about half horror and half comedy, and the problem is that the comedy wins out when it really shouldn't. It actually isn't, strictly speaking, a horror story, despite the main character being a horror-movie and zombie fan. It's more accurate to think about it as the sort of Gothic fairytale fantasy that Tim Burton popularized. The end result is an uneasy compromise: the dark-fantasy part of the show is decently done and has some clever and inventive touches, but everything else is built out of parts so standardized they can't help but creak, or break outright.
The first few episodes have great promise, at least. High-school kid Chihiro is a die-hard zombie fan, to the point that when his cat Babu dies he hatches a harebrained scheme to bring the poor kitty back from the dead (shades of Stephen King's Pet Sematary, of course). This involves a potion brewed up from a formula passed down through the family — one that has a crucial page missing, so Chihiro has to improvise and try out a whole slew of different ingredients. Night after night he sneaks away from home with Babu's body in a picnic cooler, and works his black arts in the middle of an abandoned bowling alley.
Chihiro is on the verge of giving up when one night Rea sneaks in. She's the daughter of a wealthy and powerful family, and in classic Gothic-fantasy fashion her father has all but forbidden her to interact with the outside world. But she yearns for freedom, so much so that dying free would be better than living as a prisoner, and she's drawn to Chihiro because he offers a way out.
One night Chihiro makes the discovery that hydrangea leaves, nominally poisonous, work as the missing ingredient for the resurrection potion, and Babu leaps back to life as Zombiecat. Then Rea, in despair over her father clamping down on her all the harder, swipes some of Chihiro's potion, swigs it, and after her father mistakenly shoves her off a cliff (another dependable Gothic story staple), she turns into — you guessed it — Living Dead Girl. She's still more or less herself — at one point Chihiro quips "Rea's so full of life I forget she's dead" — but zombie-dom does come with complications, like the occasional bout of flesh-gnawing (something that can be staved off with more hydrangea leaves), or the fact she has no pulse or body temperature.
It doesn't take long for Chihiro to realize Rea's unlife is now in his hands: he's the only one who knows how the formula works, and has to take steps to keep her from rotting. (Talk about a high-maintenance relationship.) What's more, he has to deal with the fact that this formerly flesh-and-blood human being is now that much more a thing — not just in the sense of being a zombie, but also being the incarnation of all his fantasies.
This is a great idea, one of the ways the show could have really expanded on its premise, but Sankarea elects to do disappointingly little with it. Instead, it mostly uses it as fodder for laughs, as when Chihiro compulsively follow Rea around with a video camera ("for science!") and films her like he's making a fetish video. She makes the usual token protests, but despite all her other maturation as a person (thirsting for freedom, etc.), she never really challenges him on this issue. It's just made into a running gag.
Most tellingly, there's no scene where Chihiro is forced to make truly hard choices about his feelings for Rea as a zombie. It's difficult for him only in the sense of the logistics — e.g., not getting caught stuffing her body into a closet when she manifests rigor mortis. The closest the show gets to actually confronting the issue is through a secondary plotline where his childhood female friend, not a zombie fan, competes for his attention. But again, that's mostly used as a way to milk cheap giggles and evoke stock sympathy.
This sort of silly byplay climaxes with an even sillier plotline where Rea's father kidnaps Chihiro, all to assuage his wounded pride over his daugher. The father, by the way, is an example of the show biting off way more than it can chew: his creepy overprotectiveness of his daughter manifested in things like him taking naked pictures of her as she grew up (!), but it's all only because he lost the one woman he truly loved, Rea's biological mother. Such psychologically gruesome material is in line with the show's Gothic roots, but it doesn't coexist well with the rest of the show's compulsion to yuk it up.
Mixing horror and comedy, or dark fantasy and comedy, is hard to get right, because one of those elements usually winds up undermining the other. Sometimes they don't even bother finding a balance: Is This A Zombie? never pretended to be anything but for laughs, which is fine, except that it didn't spend enough time actually being funny.
But often the best approach is to pick a side and stick with it. I admired how Shiki, for instance, didn't cop out or try to leaven the goings-on with easy laughs. That show also did something else Sankarea aimed for but fell short of: making evil behavior not only comprehensible but even sympathetic, and also showing how good people can fall unthinkingly into hell.
Sankarea is too creepy to be funny, and too funny to be creepy. They should have picked a side and stuck with it.