I applied to marriage immigration in July this year, and my eldest daughter was mildly mentally retarded. Can the visa officer let me through?

If you apply for immigration for your daughter together, there will be no problem, but you need the consent of your daughter's biological father;

On the contrary, if you didn't apply for your daughter when you applied for immigration, and later found out that you have a mentally retarded daughter, you are likely to be directly refused by the visa officer.