I'm talking only about STARTING NFL QBs. If you include backups, then yes, Brian Hoyer is about the Mendoza Line, and Flacco's well above the Mendoza line.
But, among established starters he's about statistically the NFL worst for the last 5 years.
And since he's passing himself off as a "former SB MVP" starting QB you don't get to compare him with Brian Hoyer or Josh McCown, Josh Johnson (Eagles), or Matt Schaub (holy crap! Matt Schaub is still in the league? Yes. Somehow Matt Shaub is still around.) Apparently Drew Stanton is too. Who knew?
You can only compare Flacco with actual starting QBs. And then, yes, he is that bad.