Feels like an uncommon to me. Leyline Phantom didn't have P/T exceeding its mana cost.

Yeah, but Leyline Phantom is really, really bad.

That's kind of the point of blue creatures.

You know how Hornet Sting is not a reasonable card even though it's overcosted, since less enfranchised players have a hard time grasping such metalevel power distinctions and will assume that damage dealing is actually a part of 's pie? How is it then, that we can support a seemingly similar idea where has 'better' creatures and has the worst creatures?

If you look at the past, it's quite clear that this standard of better/worse creatures as a whole is unmaintainable in that is hardly the color I think of if I had to list the top ten best creatures of all time from the top of my head in quick succession. Even in limited, the limitation of no flying is a really harsh since that's one of the easiest ways for a creature to have/gain evasion, and that matters for creatures of all sizes. Then there's the issue of powerkeep. How are new players, or any players for that matter, to believe the concept of having better creatures than when it can regularly get cards like Slither Blade. How do you even 'top that' in ? 1/2 deathtoucher for is arguably worse. Maybe a 2/2 for ? Seems rough to me.

Now you could argue that is more about having 'bigger' creatures. That certainly is consented, but outside of limited it doesn't really hold water. Just with the concept of iconics alone, all colors get big, fatty creatures, and usually all the nongreen ones have flying, making them slightly better by default. For a creature, stats are merely half of the equation if even that. Like, comparing the cards of the gearhulk cycle, which one is better, the or one? Verdurous Gearhulk puts a bunch of stats on the table, but unfortunately those just don't matter that much. Torrential Gearhulk has easily seen much more constructed play and that speaks for itself. On the other hand, Primeval Titan is a powerhouse, but it's power hardly stems from it's stats. That I think is a glue as to where this concept of differing creatures in different colors should be headed.

Another point could be, or is, depending how you interpret it, that 's creatures need to be bad since much of what else it has is more powerful in comparison. As I have alluded with all of this, I don't think is a particular well played out balancing act and would rather give up on it. While I don't there's any hard data for this, I would note that might be a color that has a greater quantity of balance issues that have lead to it being dominating in multiple formats without a specific intent. Maybe one of the reasons for this is this 'badness' of its creatures that's supposed to balance the color as a whole while giving it access to lots of powerful (reactive, instant speed) spells?

So I wouldn't really go with the principle of blue creatures straight up sucking - as in being artifically overcosted. As we know, power levels fluctuate so where one card might be weaker, it might be better in years to come. Also, these kind of costing conventions are pretty much thrown out of the window once we go beyond common rarity so I would question the purpose of the whole practice. In a random casual game, I doubt new players are going see the creatures overwhelmingly trumping over ones. I would rather think of an alternative route, such as making the blue creatures more specific to certain situations and glunkier that way by extension. Green creatures on the other hand could be more confrontational, proactive, flexible, and easier to use for example.

While not perhaps entirely conscious of this underlaying principle, I think I've put it to use quite well in this set. This is quite speculative/hypothetical so you are obviously free to disagree.