As Waluigi Time said, depends on how much work you want to put into the character. And if this is Ultimate you're talking about...
1. Texture edits? Very very easy to make. Just an image editing tool is required.
2. Adding armor and accessories? Easy, but requires 3D modeling and rigging knowledge.
3. Replacing a character? Difficult. Requires you rig a character to the bones.
4. Replacing a character who also comes with appropriate facial expressions? More difficult because you have to edit faces to make the separate facial expressions.
5. Adding a voice? Takes extra time but is long. It's easy to do by itself though.
6. UI? Depends. Your own render? Takes extra time. Also you need to make a stock in-style for the rest too. Renders in Ult can exceed 1000px in dimension. Replacing the image itself is easy.
7. Replacing fighter name is easy. Just a text edit in the MSBT file.
oh and I didn't even bring up moveset changes. Not sure if it's possible with Ultimate but that's another layer of work and it requires you understand how the attacks are coded.
Me: make Rivals of Aether character and I only just made a jab so far.