I don't think gross tells the whole story behind why some of those champions are less popular than others. Gragas, Trundle, and Urgot all have really weird kits that make them a potential liability to their teammates. Seriously, all of them. Gragas can accidentally help an enemy escape with his ult. You could argue that Janna can do the same, but as Gragas you're much more inclined to use it offensively and screw up a teamfight in the process. Trundle's pillar can accidentally displace allies or cut them off. Urgot can use his ult to drop himself in the middle of the enemy team and get himself killed.
Additionally, each of those champions has "utility only" abilities that make them less exciting to use compared to other champions. Gragas has his "chug," He also has trouble being a melee champion who fills a role traditionally left to ranged champions. Trundle has the Pillar, Contaminate, and his ult. He's also mostly used as a jungler and while he clears fairly quickly for a single target champion, he's no Shyvana or Udyr. Urgot has the shield and his ult, and his kit is a weird hybrid of bruiser and carry that makes him hard to place on a team. I'm not saying they are bad champions or bad abilities by any means. They're all extremely useful, but in my opinion being heavy on utility (moreso for utility that only benefits you) and having weird kit design for their role makes a champion less appealing in solo queue when you need to depend on teammates who might not realize that Trundle isn't the same as Lee Sin and Gragas isn't the same as Ahri. Given a choice between the guy with lots of damage and/or hard CC and the guy with utility, I prefer damage and/or CC unless I am playing with friends who I know can take advantage of my utility. In fact, I would say that their "grossness" makes their kits more appealing, because those kits on "pretty" champions would be a bit less interesting.
Now compare those guys with Cho'Gath and Kog'Maw. Kog'Maw has an awesome kit for an AD carry. He is somewhat dependent on his team for protection, but that's not exactly different from any other AD carry. He lacks a dash but his slow serves a similar purpose. Cho'Gath has two forms of CC and a true damage ult, and can be built super tanky or as a damage dealer. He's not the best top or jungle in the game, but he can certainly hold his own. They're both pretty useful without being team dependent, which in my opinion makes them more appealing choices than the other "gross" champions.