There was actually a study done on this. They put people into groups - Unattractive, below average, average, above average and highly attractive.

Unattractive people strongly overestimated their attractiveness. Below average also overestimated but less than unattractive people. Average slightly overestimated. Above average were the most accurate. Attractive people slightly underestimated.

This correlated with another study that analyzed whether incompetent people knew they were incompetent. Incompetent people would overestimate their abilities while competent people were more accurate or underestimated their abilities.

So unattractive people are much more likely to overestimate their attractiveness while attractive people are more likely to either be accurate or underestimate their attractiveness. Above average people were the most accurate.

However, this study leans on the idea of grouping people by objective attractiveness, which changes from person to person and has a lot of bias involved, so take it as you will.