At the time of writing this review we have exactly two 1 Cost allies in the game's pool of ally options. Blade is the other one, and both of these cards are incredible as long as you can meet their requirements. This small pool of options however makes it hard to compare Stinger/Cassie Lang directly to other identical cost allies.
That written; we can see from the 2 cost allies that for an effective card cost of 3 the typical statline you get for your ally is 1THW, 1ATT, 2HP and some other upside. There are some exceptions to that, like Ironheart's refund or Hulk's incredible statline but on the whole it is fair to say that Stinger is on par with an average 2 cost Ally for any Leadership based deck for an Avenger traited hero. The absolute floor for her is; she can enter play, attack or thwart for a point and then chump an attack. However, when you start to utilise her specific strengths you'll notice the ceiling for this card is very high.
Stinger not counting against the ally limit is a bonus that's as large as she can sometime grow; and we can highlight just how massive that is by looking at her ability through an alternative lens. Cassie Lang is effectively an ally with a copy of The Triskelion printed right there on her text box. Except it's not actually The Triskelion at all; so you can use both her alongside the support card to reach five allies for Avengers tribal decks. In short, Cassie comes bundled with approximately 2.5 cost worth of value. She is not strictly better than any other ally, but not counting against your Ally Limit is a huge deal, especially for ally swarm style Leadership decks.
I would not categorise Cassie as a staple for all Leadership aspect decks, but for tribal swarm decks using the Avenger trait and looking to use cards like Strength In Numbers, Team Training, Mighty Avengers, Avengers Assemble!, Lead from the Front, Mass Attack, All for One and Band Together this card is an automatic inclusion even if she will rarely be used as a chump blocker in decks that are looking to go wide because of that innate The Triskelion makes it worth keeping her around if you care about having as many allies as possible in play.