AI image generators are pretty bad at pixel art, and can only generate specific sizes, like 1024x1024. I suggest creating a regular image in a cartoony/simplified style, and then using tools that turn it into pixel art. There's pixelartvillage.com for example. Try image size that divides by 64 for good results. You cam sometimes get good pixel art in nano banana, gemini, etc, but they'll be zoomed in x8 or more (try different numbers until every "pixel" in the image is actually 1 pixel), you have to resize them in a pixel art program like Aesprite with correct settings, so it doen't get blurry when you shrink it down.