The really big fan push was behind Shannon Purser, who played Barb in Stranger things.
Which actually seems to have been part of what got the project green lit. Innernets was all “Barb for Squirrel Girl” well before it looked like Marvel was going there. And there were reports she was in the running. She very publicly pursued it.
She’s the right age, funny. And was apparently already familiar with and fond of the character.
Which made it all the weirder that they eventually cast some one in their 30’s.
Anna Kendrick also took a run at it once it was announced. Which would have been fun, she’s 36 so that may have been part of Marvel’s shift older.
ETA: for the record I think any and all of these could work depending on the project and tone they shot for. The thing with Purser is she seemed to fit really well for a close adaptation of the recent comics, as well as slotting into the current MCU. Which is absolutely not what any of these other choices or the actual show they attempted fit.
