Not necessarily. The entire current MCU started with a fringe character--Iron Man. Casting and story matter. Crap like The Marvels was complete and utter garbage--Capt Marvel is a completely unwatchable character and it's not because she's a woman. Gal Gadot was a fantastic WW and Scarlett Johansson was great as Black Widow. Casting and story matter...don't force junk on the audience and we'll watch. Heck, we'll pay good money to watch.
I'm a YUGE DC fan, but I'll wait for free streaming for Super Girl. Why? Casting is terrible.
Iron Man definitely wasn't fringe. I grew up during the "action figure" heyday (Star Wars, GI Joe, He-Man, Transformers, etc.) and was heavy into comics, as well ... and Iron Man was well featured even back then. Only the "top" guys in the comics were getting action figures, and he was one. Spider-Man, Iron Man, Hulk, Wolverine, Captain America ... those were the top cats for Marvel then. Iron Man was actually my favorite.
But you're correct that casting and story matter ... but so does picking the right characters.
It's not a shocker that, going forward, for Marvel to be successful, I think they need to focus on the bigger characters, and when they do go a bit fringe, they need to bring in a stud actor and have a killer storyline that matches that person's vibe. Like they're having a go at Ghost Rider yet again ... and the first thing I thought of was "they need a Ryan Gosling" type to make that work ... and that's who they got. You give him the right script, and that could work ... they're also casting Sacha Baron Cohen as Mephisto, and that has the potential to be a killer casting choice, if used correctly in that movie ... though his debut as Mephisto in Ironheart left a lot to be desired (too straight/serious).
NOTE: Marvel probably would have started their movies with Spider-Man, or X-Men (for Wolverine's popularity), but they had sold off the movie rights to those 2 properties, so they chose 2 of their 3 next most popular ... Iron Man and Hulk.