According to the comment, James Gunn knew that Stan Lee was getting older and wanted to make an easy appearance for him:

“I originally wrote Lambshank, the character I played, for Stan Lee. […] I knew he was also getting a lot older and it would be harder to bring him to Atlanta to shoot, so I made a character that I could just animate and then have him do the voice and have a little face à la Stan Lee on this character. But sadly Stan passed away, who I’ve really enjoyed working and directing with so many times through these films.”

This character ? It was Lambshank, one of the hybrid creations of High Evolutionary (Chukwudi Iwuji). If you recall, at the end of the movie, Mantis (Pom Klementieff) is horrified by the twisted creature but tries to tell him it’s because he looks so cool. Lee could have recorded the cameo from another location. Gunn ended up playing the part instead.

On a personal note, I used to co-host a web series called “Cocktails with Stan” where he and I interviewed celebrities. One thing Stan always talked about was how much he loved doing cameos. I joked about doing a historical movie once backstage, and he immediately said, “Ooh, what cameo can I do?” He was so excited about his appearance on the TV series “Agent Carter” that he broke the news to me and told me to write it down at the time. He would have loved this one.

To that end, his company Pow! Entertainment has reached an agreement with Marvel, so that his likeness and voice can be used (via THR) in future television and film projects that work with “who he was”. He would have liked that too, I think.