Morgan Stark initially had a cameo in Spider-Man: No Way Home that was ultimately cut, and that's probably for the best. Lexi Rabe, who played Tony Stark's daughter, Morgan in Avengers: Endgame, has revealed she shot a cameo scene for the latest MCU film. While it would've been nice to see Morgan again, it's good that this cameo was taken out of the final film, as it probably would've had a negative effect on the end product.
The addition of Morgan Stark in Avengers: Endgame was a welcome and surprisingly emotional addition to the MCU that offered plenty of potential for the future of the Stark legacy in the franchise. Considering the character of Peter Parker is very closely associated with that of Tony Stark, it would make sense to add Morgan into the mix of No Way Home as a way of showing her importance to the future of the MCU as a whole, and perhaps the connection she and Peter may share. Tony Stark served as Peter Parker's mentor throughout his first solo film, Spider-Man: Homecoming, as well as in Tom Holland's other appearances as the character in Captain America: Civil War, Avengers: Infinity War, and Avengers: Endgame, so in concept it would make sense to establish a connection between the Spider-Man and Iron Man franchises. However, it was to No Way Home's benefit that the choice to remove Morgan Stark's cameo was made.
A simple reason as to why cutting Morgan Stark's cameo is a good thing is that Spider-Man: No Way Home is a very crowded film already, and she would've been too distracting. Seeing as No Way Home is already balancing five villains on top of its main hero and those around him, there is just nowhere to put a Morgan Stark cameo that wouldn't be distracting. No Way Home is a Peter Parker story first and foremost, and the film is great at balancing all of its many characters while maintain its main focus on Peter. But with an extra cameo from Morgan Stark thrown in, that balance is broken, and the film's overall narrative would feel messier as a result.
Another reason as to why Morgan Stark's presence in No Way Home wouldn't work is that she and Tom Holland's Peter Parker need to have their own distinct futures within the MCU. A general complaint some had with Tom Holland's first two Spider-Man films was that he was constantly in the shadow of Tony Stark and didn't get to find his own identity, whereas No Way Home is about Peter reclaiming his identity and learning from himself rather than Tony. Keeping the Stark legacy a prominent focus in Holland's Spider-Man films at this point would be robbing the character of his own future, as well as Morgan of hers, as she needs to walk her own path going forward as well.
Morgan Stark as a character is acting as an exciting branch of possibilities for the future of Tony Stark's legacy in the MCU. But while she will surely make a great addition to the MCU some day, Spider-Man: No Way Home is Peter Parker's story, and is an important turning point for Tom Holland's portrayal of Spider-Man. A Morgan Stark cameo would've just thrown a wrench in an already dense and personal story, rather than been a fitting addition to the film.