Even though their fundraiser may have reached its target, the new citizen needs some time to pack and get ready to move in! If you've helped a fundraiser reach its goal, the new citizen will move in on the last day of the fundraiser period. For example, if you contribute to Dorothy's fundraiser when it starts on on 11/01 and the fundraiser period lasts for 14 days, even if you reach 100% of the fundraiser goal on 11/05 you will still need to wait until the last day of the fundraiser period on 11/15 before you will see Dorothy appear in your city!
Note: If you reach the fundraiser target ahead of time, you can still continue contributing to the fundraiser. The more you exceed your fundraiser's target, the more special rewards you'll receive!