Shemeska
Adventurer
We are never told opnely that the Lady is unilaterally more powerful than the gods, it's only inferred from her ability to keep them out of Sigil. But occam's razor doesn't always lead you to the right conclusion. Sigil may just afford her that one trump card that prevents her from having to confront any gods.
It was the story of Aoskar the now dead god of portals and planewalking that really brings home the idea that the Lady is on some level more powerful than a god. But Prince Levistus isn't a god and he forcibly shrank the size of the deity Set's domain (which is inside of Levistus's layer of Hell). It might be something like a home field advantage. Her Serenity might be utterly powerless outside of Sigil, we just don't know.
Here's a little something related to deities wanting to get inside of Sigil, and ways The Lady handles the issue rather than simply blocking them entry.
[sblock]And I'm suddenly taken by the picture of an elderly barmy huddled under a makeshift shelter in the Hive, clutching a blanket around his shoulders against the faintly acidic rain, rocking back and forth with a wild look in his eyes.
A group of tieflings wanders past and they hear him muttering to himself, "Almost time almost time almost time... cannot call them back before it's time... not yet not yet not yet..."
One of them pauses and against better judgement leans in and asks the poor old fool what he's talking about. The old barmy gets a wild gleam in his eyes and works himself into a fit. "I'm a god you know! I'm a god! When She's not watching, I'll make Her pay! You'll see! Just you wait!"
The tieflings laugh at the tramp and leave him sitting there in the rain, still muttering to himself, lost in his own delusions. A block away as they wander into the Bottle and Jug, away from the rain, one of the tiefers recalls something his grandfather told him years before about a crazy man just like the one they'd seen, a crazy old man that he'd seen, and his grandfather before him had seen. A thousand years had passed since that nameless man entered Sigil with dreams of power and glory, needing only to wait for the moment that Her Serenity no longer watched him, needing only to wait to call to his proxies to return his divinity and restore him to his deific glory.
A thousand years had passed, and so had his proxies. One by one they'd abandoned their god, taking his divinity for themselves, or delivering it to other rival gods, some even selling it like the choice sweetbreads of infants to any archfiends willing to pay for the chance to dine on the fragments of divinity. A thousand years had passed and he had nothing left but the eager hope of toppling The Lady, but oh he had to wait, he had to wait until the time was right, and then he would be the victor.
You see, not all Mazes are physical things. Not all Mazes are imposed from without.[/sblock]