She wasn't even important enough to appear in the finale.
I want Fireside Girls!
Who cares about that when we have Gretchen?
>Marvel characters exist in Phineas & Ferb universe
>Phineas & Ferb universe is part of the Marvel multiverse
>Marvel multiverse was destroyed by the Incursions
>Implying they wouldn't be able to build a vessel capable of bringing the entire city of Danville into the new universe in time.
You forget those two can give any Reed Richards a run for their money.