Aseprite has no pen pressure support, so the only benefits from a tablet would be the alternate pointer control and any hotkeys you get built into the tablet (which is none on an iPad, but you do get some on an Intuos).
Since the benefits are so small, I don’t recommend buying an Intuos unless you also plan to use it in other drawing programs that can make fuller use of its features.
If you have the iPad already, then it’s worth trying out! You might like working that way, you might not, it depends a lot on your working style and the type of pixel art you make. For some kinds of pixel art, a mouse is the ideal tool.
Edit: Aseprite has since added pressure sensitivity support. While a tablet is far from necessary for pixel art, it does allow one to work faster, especially while sketching.