Erdrick isn't nearly as recognizable as Cloud, but he is tied to tons of DQ games, unlike how Cloud is. They're different series with a different type of lore. He's the most recognizable protagonist, for sure. The thing is, beyond that, he's not as much as like Cloud as he could be. He's not the actual series mascot. That's Slime specifically. He's easily the secondary mascot. Right now, Cloud has become the actual proper mascot of the series. He was more like Erdrick at some point, very important, but wasn't the series' mascot yet. He took over that role from Moogle/Chocobo, the original mascots of the series. Erdrick can't really take that role away from Slime, but he doesn't have to. He's still more than important enough and doesn't need to be the series' mascot to be known.
Now, let's be fair; Erdrick is a literal who to many. There's no denying that. Cloud has a minority who doesn't know who he is in comparison(of course, neither are on the level of characters like Pac-Man, Sonic, and Mario. Slime actually comes really close in iconicness, though. Like, who doesn't even know what a Slime is, not even the DQ one specifically. That's the cultural impact one little monster had. Erdrick didn't actually do that. His descendant in the first game is the one that paved the way for the avatar heroes to come. Erdrick's is just more important to the series' lore, not so much when it comes to helping impact later games. It's two different roles for two major characters. And Slime on a different role, respectively). However, despite being not nearly that recognizable to many, he's still a very major character in gaming and is more than deserving.
The reason why Cloud became the only protagonist in a single FF game to become a proper mascot is because FFVII heavily impacted gaming. Other DQ games too, sure. That much is true. I mean, it's the most influential rpg series in history(not just video games. DQ references are everywhere. "A Wild Monsters Appears!" is even a Yugioh card, which is a Manga series and all first. Obviously Link owes more to Erdrick's descendant, the first hero shown, than any other video game character for influence. There's some apparent thing that he also was influenced by Peter Pan, but I don't remember if that was true or not. It does explain the Fairy stuff, though).
Erdrick, that said, absolutely has become the only protagonist in DQ to become a mascot of the series. He's still behind Slime, of course, but as I noted, that's not a problem at all. Not for Smash, which always represents a series with the first entrance being either the de facto mascot of the series, or a mascot of a game in the series. They're both great.