Overall, "Fowl Weather" is a nice showcase for Iago, even if the character arcs do have some faults, and yet another example (as are the majority of episodes, really -- I suspect this will be be an aspect of most of my future reviews) -- of the series' flair for world-building ... or, more appropriately, world expansion.
Some episodes, or at least certain elements of such, have a historical basis (as we saw in "Getting the Bugs Out" with the Greek aesthetic of Mechanicles' home base and garb), while others indulge in pure fantasy (as did "Mudder's Day"). The premise of a tropical valley from whence all of the planet's rain originates that is the domain of an avian goddess who directs and controls said rain has a decided mythic bent. As such, this is a more whimsical and -- despite the furious Thundra's assault on Agrabah -- light-in-tone episode. The "phantasmagoric" mud beings and their colorful realm in "Mudder's Day" certainly exhibited their share of whimsy, but with the heroes' captivity in the Al Muddi Sultan's palace and his attempted Godzilla-like rampage on the surface, that episode had a more perilous, sword-and-sorcery, heroic quest angle.
Anyone with a cursory knowledge of history knows that many ancient civilizations and cultures had a pantheon of specialty gods. Many of these did indeed include a rain deity, although some fleeting Googling hasn't turned up any of an ornithological orientation. Thus, as far as I know, Thundra is a fairly original -- and conceptually well-defined -- creation. True, her snobbishness, temper, and her reactionary hostility toward her visitors are one-dimensional and prima facie, but it gets the job done in regards to carrying out the plot. Her Romani/gypsy accent (complementing the garments she's adorned herself with) is a bit over-the-top, but at least it exemplifies the creative team's continued dedication to locales and cultures of the real ancient world, even as part of such a fanciful palette. Or, it could just be the creative team falling back on a stock caricature type.
Unwittingly finding himself the object of Thundra's passion, this isn't so much a character-defining or character-building episode for Iago as it is just an episode in which something happens to him in particular and he reacts to it. In fact, the whole reason Thundra's a bird may just be for the purpose of "something happening" between her and Iago! He's his irate, recalcitrant, grumbling self throughout, and I, for one, enjoy the angle he adds, as I always do. His revulsion toward Thundra and protests against her advances are highlights, and ring true to character and are sympathetic, given how overbearing Thundra is. But I don't buy his softening toward her when she descends upon the palace -- it seems like a forced way to bring about a resolution and to vindicate and not contradict all of Jasmine's "it's wrong to mess with a woman's heart" admonishing. On the other hand, Thundra's "a rain bird's work is never done!" declaration is a clean out in terms of the writers getting her out of the picture and keeping Iago at home for the rest of the series.
Jasmine's aforementioned objection to the boys' encouraging Iago to lead Thundra on so that they can make off with a storm cloud while she's distracted is not only forced and preachy in delivery, but it's a fairly transparent case of the writers finding a role for the one of the main cast members who otherwise would just be along for the (carpet) ride. Still, it's a necessary one, and for the deceit not to be addressed and to have been maintained through to the episode's close would've made for a sour note. While there's a certain logic to Jasmine identifying and sympathizing with Thundra, was it really so necessary to play it up as a gender-dividing matter, as if it's a "guy thing" to play others for suckers and a "girl thing" to object to such?
The production values are notably lesser than the preceding two episodes and the OVA before it. The backgrounds are stark and blunt. Even the animated storm clouds and the thunder bolts that they generate are minimal and underwhelming. This is even the case throughout Thundra's vengefully turning her wrath on the palace, which should be a cataclysmic spectacle, but comes off nondescript and even casual. In fact, upon further consideration, the rudimentary visuals, along with the sunniness of most scenes, may account for the episode's light-heartedness more than anything.
Don't think that (quite) all is fun and games, though; the peasant boy seen at the very beginning and tail end and the gang's efforts to aid him during the drought are an exception. They bring a down-to-earth realism and sense of palpable need to the episode, much like the villagers in "Getting the Bugs Out" and the character interplay in "Mudder's Day".
Genie watch: Overall, he comes off better than usual. He has some extended impersonation bits -- a TV weatherman and a door-to-door vacuum sales cleaner -- that find him reveling in the role and function for which he's best-suited. He even is helpful at more than one turn, being the one to inform the gang of Thundra's valley and propose it as a solution to their drought woes, and later using his powers to eavesdrop long-distance on Iago and Thundra, to monitor if Iago's keeping the ruse going. However, they could've found a gag to carry his failure to hit water when in the form of a drill bit, hoping to help the little boy, without making it look like a result of his incompetence and making him look so undignified -- after all, it WASN'T his fault. His one true "dumb" moment of derailing the gang's efforts and taking an unnecessary amount of time to recovering, though, is when he crashes into the rain forest while attempting to rocket-propel Carpet over the valley. Oh, and he's not particularly helpful when the palace is flooding and the roof is caving in, but at least he's not shown actually TRYING to do, and thus not botching, anything.