Yeah, I'd say it's pretty believable. Especially since I recognize a lot of them.
Is Metropotamia's flag based on Washington County, MD? It seems like there's a lot of resemblance. [link]
I also like your Washington State's flag. I can see how it might be derived from George Washington's coat-of-arms, yet it's incredibly distinctive. It actually reminds me more of DC's flag than the coat-of-arms, for some reason.
I also like Massachusetts's flag-- it reminds me, as it's probably supposed to, of the old New England ensigns. And it doesn't have a full-blown seal, at least.
I also like Vermont, the general lack of seals-on-blue, even if there are seals-on-other-colors, and the... non-rectangular shapes.
I do wonder why there's so many flags based on Texas's-- I was under the impression that those only became really popular after the Civil War, and partially as a result of it?
I also wonder exactly how your history with Mexico went-- why they kept California, for instance, but not Texas and New Mexico. Speaking of which/vaguely connected, I like Georgia's Gadsden flag variant.
I approve of the fact that you kept Maryland's basic design, but I wonder why it looks less like it's being flown upside down than like it was inserted into paint ot something and flipped. If it were just upside down the vertical stripes would be yellow-black-yellow-black-yellow-black. For an actual, physical flag like this you'd have to attach it to the pole at the fly and let the usual hoist end flap around. I mean, it came from the Calvert/Crossland arms. Why would it get turned around in the first place?