The title for "Cupid's Dagger" tells you a good deal of what you need to know about this episode. It's in a long tradition of farcical comedies about people falling in love for reasons outside of their control, leading to broad silliness and embarrassment. Usually the question for a show like this is: Did I laugh or did I cringe? Maybe both?
Indeed, I saw this plot just earlier this year with Grimm's "Blind Love," which was a fun example of good-natured silliness as a humorous detour. It made me laugh, especially when a character fell in love with himself and sang into a mirror. That was a good twist. On the other hand, the cringe example that always immediately springs to mind is DS9's "Fascination," which was just an awful collision of characters rushing to the front of the line to embarrass themselves.
"Cupid's Dagger" is probably closer to the cringe example, but nowhere near as bad as "Fascination," and with a few laughs that emerge from one notable twist in the scenario. Now, you must accept this on the level of "extremely silly" for it to work at all, which is fine. But it would probably be better if it didn't unfortunately lean into some of the worst ongoing character material on the series — namely the rehashing of the reasons behind the Ed/Kelly divorce, which this story all but revolves around, and Yaphit's borderline-stalker-like pursuit of Claire. Meanwhile, it features a placeholder "mediating two warring third parties" backdrop that is perhaps the grandest of all Trek cliches. (Yes, the plot is a means to the comedy's end, but that doesn't make those scenes any fresher.)
We've got two races who have been at odds forever over ownership of a planet. A recently discovered artifact may prove via DNA testing who the rightful heirs to the planet are. An archeological expert is called in to do the test that will resolve the dispute. That expert turns out to be Darulio, the blue man whom Kelly cheated with on Ed back in "Old Wounds." That Darulio is played by Rob Lowe in his affable I'm-just-being-me-and-not-looking-to-hurt-anyone mode (think Parks & Recreation) probably saves this episode from a great deal of unpleasantness. Lowe is good casting, creating a likable guy instead of someone who could've easily come off as a total douche.
But rehashing Kelly's affair and all the histrionics that go with it (didn't we have to sit through this bickering in "Pria" already?) is an enemy to comedy, drama, and entertainment in general. It feels like it should be beneath these characters. It's cringe-inducing material, and not in an effective Office or Curb Your Enthusiasm kind of way. It's merely dumb and pointless and makes you feel sorry for actors being asked to wallow in such material. (Well, you don't feel sorry for Seth MacFarlane, since he's the one who allowed it in the first place.)
Things get worse before they get better. Kelly sleeps with Darulio again, and Ed walks in on them again, and Darulio explodes blue liquid from his face again. Har har. But it's clear even before it's explicitly told to us that Kelly falling for Darulio is because of sci-fi reasons. (Darulio goes into heat once a year, and his pheromones make anyone he touches fall insanely in love with him.)
The twist here, which makes the episode bearable, is that Ed finds himself falling for Darulio, which leads to some funny scenes where Ed spends time in Darulio's quarters and just stares at him in awe, and fawns over him with cornball lines. And it's funny to watch the focus of jealousy shift among the vertices of the Ed/Kelly/Darulio love triangle. This seems to be much more in MacFarlane's wheelhouse as an actor, and he throws himself gamely into these scenes and is much more natural playing them.
As for the material between Claire and Yaphit, the less said, the better. Watching Clarie fall under the influence and throw herself at Yaphit feels more like "Fascination"-level character humiliation than humor, and all the shapeshifting goo-sex that goes along with it felt dumb and unseemly rather than funny. While I know there's no ill intent behind this plot beyond goofiness, watching Claire throw herself at someone who has basically been sexually harassing her in most of his scenes is questionable, especially given our current state of heightened awareness for sexual harassment in the entertainment industry. Bad timing, I know. But even apart from the timing the implications are there.
And you have to wonder why Darulio would allow this to continue without explaining himself. This is something that happens to him every year, so doesn't he have some responsibility for being proactive about preventing it? (I know; asking logical questions where they aren't wanted.)
As for the predictable application of Darulio's gifts on the ambassadors to stop a war and make everyone just love each other — well, it's an obvious move you can see a mile away, and it plays as silly, but it's not exactly a believable solution, even for a few days while the true ownership of the planet is sorted out. Hey, whatever, man. We're not supposed to care about any of this, which makes one wonder why the story spends time on it.
"Cupid's Dagger" is silly and light and dumb, and that's what it's going for. But it's only sometimes funny and it requires you to sit through too many things that are annoying in order to get to the good stuff. Since this is a straight-up comedy episode, your mileage may vary.