Popularity of the character only means so much. Batman is even more popular than Spider-Man is and it didn't save Batman & Robin from flopping or Batman Begins from underperforming.
Now I don't think anybody is predicting that ASM will be a flop, but is it likely to do worse than both the previous Spider-Man films and the other two big superhero films coming out this summer? Sure looks like it right now.
There's a pretty good argument that Batman and Robin wasn't a flop. At least not financially. Which goes to the point that brand does matter.
Spider-Man is a recognizable brand, so at the very least it's a safe choice for parents to drop their kids off at the mall and pick them up 2 hours later. Like Ice Age and Madagascar. But for TASM to reclaim the peaks of the Raimi franchise, it has to sell itself a lot better than it's doing to date. Right now it looks like it's going to be a distant third to the other two superhero franchises, at least in America, it has none of the critical buzz of Prometheus, and nobody in the general public seems excited. There's more wrong than just SM3 wasn't well received. TASM might do as badly as a competently made Spider-Man film can do.