What are Squirrel Girl's 'actual' stats? Is she truly the most powerful being in the Marvel universe?

Squirrel Girl's stats on the Marvel Wiki are not considered to be canon. As with all entries on that site they were written by fans rather than being a curated collection of facts written by Marvel staff.

In this case, her article and stats were written by a comic-book fan named Kyle Nichols (aka Mimicx35). He's contributed to multiple articles but I see no evidence that he has any special relationship with Marvel other than being a keen reader of comics and running several now-defunct comic chatrooms on MSN in the 1990s. A Marvel staffer did stop by to make a cosmetic change to her article (adding a better quality photo) but that's about it.

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Her actual stats, as seen in the Official Marvel Handbook of the Universe - Avengers 2005 edition are those that are considered to be canonically valid and, crucially, actually fit with what we actually see her doing in the comics.

Intelligence - Human normal
Strength - Superhuman (lift 800 lbs-25 ton range)
Speed - Superhuman (peak range: 700 MPH)
Durability - Enhanced
Energy Projection - None
Fighting Skills - Experienced fighter

These are replicated in a later edition, the 2008 Official Handbook of the Marvel Universe A-Z (2008) where she is described as a "surprisingly fierce and formidable unarmed combatant".

And again in the Women of Marvel: Celebrating Seven Decades handbook (2010).

along with a brief description of her strength and agility

If I had a Marvel wiki login, I'd edit these into her article.