When Deadpool was designed (by Rob Liefeld, who more than deserves the "Iron/Dark Age Incarnate" tag) sure... he was basically a rip-off of Deathstroke over at DC (down to the "in-joke" Wade Wilson name) made even more "xtreme" and gritty. But at that point Deadpool was a minor supporting character who showed little to no personality and was largely "overly badass henchman of the week". It was only really when Fabian Nicieza got a good hold of him, started to give him a personality and used him to mock the rest of the late 80's early 90's grim-dark super badass characters that he started to take off... and it was when Joe Kelly ramped all of that up to 11 that he became popular. Arguably it was only really when Fabian Nicieza got hold of him again for the generally excellent Cable vs Deadpool series (which spent quite a lot of its time ripping apart that late 80's/early90's period and the cliches that came with it) that Deadpool really became super popular.
As for why he's popular? I don't think it's the Iron/Dark Age elements at all... it's how entertaining he is in the hands of a skilled writer with his idiosyncrasies, fourth wall breaking and general humour. Iron/Dark Age heroes generally tried (emphasis on "tried") to be some awesome wicked-cool badass; Deadpool loves Golden Girls and is scared of cows and chickens... if he's cool it's in a deeply ironic way.
No. Deadpool is 'funny' because he's crude, crass and pokes fun at the idea of Comic Books. He's supposed to be a mirror, and he makes mention/mocks some typical tropes in a snide 'knowing nod' kind of way. Kind of like Joss Whedon, with his constant nods at how much smarter we are than the 'medium' of whatever style of show he's making that year. Deadpool points out how stupid comics are and how dumb we are for liking them, without pointing a finger. Letting us laugh at ourselves.
Which, frankly, sucks the entire fun out of reading comics for me. I don't care that comics are stupid, silly fun. I like them because they can be excellent story telling tools, morality plays, or just a fun, silly and/or downright stupid story to pass the time with. Just like a good book, or TV show or Movie. What I don't need is some writer coming along, thinking that we're all idiots and we need a character to point that out in a 'light hearted way'. Mark Millar and his Next Wave or his Wanted is more than enough, thank you.
What does that affect with my personal wishing of writing a dark/iron age style 'comic' story on E? Because any fans of Comics will know that Deadpool comes from that era, and because he's 'funny', his 'image' will infect anything that anyone mentions is Iron Age in style. I see it happen locally, and I have a hard time seeing that it's just my circle of literal comic book fans, who I've never met passed meeting once or twice in the local comic shop.