Well, if you define a flop as poor box office, then that's wishful thinking. Critically though, you could be on to a point. Personally I wouldn't be able to see far enough past Tom Hanks and the dwarf silverback gorilla he wears on his head to give it a verdict.
I have to say though, I'm not the least bit interested in the film. I read the book believing the hype that it was a look into the shadowy world of secret societies -it says right in the prologue that Opus Dei does exist- only to see a Tony Robinson documentary shortly after which not only denounced just about everything in the book as bull**** (including Opus Dei, which in fact doesn't exist and was a prank by a bunch of Frenchmen, debunked many years ago) but as being ripped straight from Holy Blood, Holy Grail. This was a *long* time before the lawsuit, I might add.
Anyway, take away the gimmick and you have an incredibly average thriller. Any bestselling paperback thriller you can pick up at an airport has better-quality storytelling in that regard.
Lastly, Tom Hanks is probably the last person I'd envisage in the main role. Not insulting his acting skill (though I do think he's a tad overrated), but the book describes a slightly more bookish Indiana Jones, which The Forehead plainly isn't.