Squirrel Girl is better than it seems, but only in some instances. First of all, she is an avenger so is good for some interactions. She is also cheap, and that is useful too in some decks. Her ability gets better in some scenarios like Ultron, making her a multi dron killer or versus Zola or another villian with heavy though minions taking multiple tough cards away at once.
Response: After Squirrel Girl enters play, deal 1 damage to each enemy.
Our absolute baseline floor for Squirrel Girl is 3 effective cost for a 1 ATT/1 THW/2 HP ally that deals a damage to the Villain when she enters play from anywhere. That's 2 damage split 1 and 1, or 1 damage and 1 threat removal and also a chump blocker. She's not on Maria Hill's level, but that is a very high bar to clear for any ally - especially one that can knock Tough off villains and minions alike.
Her largest benefit is sweeping damage against many minions; which makes minion heavy scenarios the ideal place to use her, with Ultron's Ultron Drones being the ultimate target to use her against. She also has an ability which still works when returned from discard via cards like Make the Call or Rapid Response and she's cheap enough to be a great target for Regroup.
Another huge bonus is that low cost of 2 and the Avenger trait which means she does not only fit in Leadership recursion decks; she also has a place in Avenger based Swarm decks that want as many allies as possible out at a time alongside each Avenger ally also having high impact when they enter play. She's great for cards like Strength In Numbers, Avengers Assemble! or Mighty Avengers.
Also, she scales very well with the number of players because of her wording; the more players out, the more minions that are going to be around for her to ping and possibly defeat. So she has wonderful synergy with Aggression team mates that like to drag out lots of minions and she is a superb ace up your sleeve to help anyone in a pinch as it is not uncommon for minions to just have enough health that they survive an attack with one health. She's a lot of "attacks" for such a low cost.
In short it is rare that I do not want to have Squirrel Girl in a Leadership deck, and I will automatically include her as a Staple when I start deck building; the kind of decks she gets cut from are rare and tend to be other Tribal decks like Guardians or Web-Warriors.
Final Verdict: Leadership Staple Ally alongside cards like Maria Hill, Beast, [Stinger] (/card/12014), Kaluu or White Tiger. Instead of needing a reason to include her in the deck you instead have to have a reason to be not playing her.