Howard wasn't a super-hero, so it makes sense that he didn't fight super-villains. Or at least, not real super-villains - who will shed a tear for Pro-Rata the Cosmic Accountant or the Space Turnip? The only Howard villain with any kind of staying power was Doctor Bong, and that's not necessarily because he's a good villain. Doctor Bong is one joke. Certainly, many more successful comic-book characters have been less than even one joke, but still. Doctor Bong has a bell on his head and his hand is a clapper, and his name is drug innuendo. If you can get that, you've got the character.
It's worth pointing out that perhaps the major weakness of Gerber's initial run on Howard - and even some of his later Howard stories - is an almost pathological reliance on parody. Howard himself began his career as a type of parody character. (Disney later made the legal argument that Howard was wholly derivative.) It's hard to create work of lasting significance within the narrow parameters of parody: by design the genre is ephemeral. Anyone unfamiliar with Howard who goes back to re-experience the original run for the first time might be tempted to dismiss the series based on just how many dated parodies there actually are - Star Wars, the Reverend Sun Myung Moon, Master of Kung Fu. But this was how Gerber worked, for better for worse: he invariably began with blatant parody, and proceeded inward from there. Oftentimes, reading Gerber's work, you get the feeling that he's trying to outpace his conscious mind. All the truly sublime moments in Howard's run are built on off-hand character moments, inserted tangentially in the context of elaborately silly parodies and satire. Gerber, when writing Howard, couldn't just start with the sublime and work his way outward. He had to approach it from an abstruse angle.
Howard's adventures usually followed a pattern: weird things would happen, more weird things would happen, until Howard became indignant and exhausted. The underwhelming parodic elements in his adventures were underwhelming to Howard as well as the reader, and the indignation he felt at being the unfortunate victim of such a long strong of useless adventures acted as a mirror for Gerber's own exhaustion. Gerber was grasping for something bigger than could be readily accommodated in the pages of mainstream American comic books. But then, paradoxically, the context of super-hero books was absolutely essential: if Howard had ever found himself in a world of peace and contentment, a world without struggle or humiliation, he would have had nothing against which to rail. Similarly, if Gerber had not been forced to channel his energies into the merciless confinement of mainstream Marvel, from where would his inspiration have come?
Of all the wannabe and never-were villains who crossed Howard's path, the best was without a doubt the putative "Kidney Lady". Really nothing more than a crazy bag lady who rode the bus back and forth across Cleveland all day, she became fixated on Howard for his supposed danger to the purity of human kidneys. There was some sort of conspiracy at work, apparently. Anyway, it was revealed (in the above sequence) that she had been the victim of a callous one-night stand during World War II. That was it. I think Bill Mantlo later tried to establish that she had some kind of legitimate magical powers, but Mantlo's work for the Howard the Duck magazine is best ignored. (Although, it must be noted, the Mantlo-created Ducktor Strange showed up in an issue of She-Hulk a year or so ago, and that was kind of fun.) The Kidney Lady succeeded in having Howard institutionalized - it was in the mental hospital where he met KISS, and (I believe) the Son of Satan. But it is somewhat telling that the biggest threat Howard ever faced was really nothing more than a broken-hearted and delusional bag lady, whose base of operations was Cleveland Public Transit.