- Son feos (masculine)
- Son feas (feminine)
- Son poco atractivos (masculine and feminine)
- Son antiestéticos (masculine and feminine)
"Feo" and "fea" are the most direct translations and can be used in informal settings. "Poco atractivos" is a less direct way of saying "ugly" and may be preferred in more formal contexts. "Antiestéticos" is a more technical term that means "not aesthetically pleasing" and can be used in both informal and formal settings.
It's worth noting that the word "feo" can also have a metaphorical meaning, referring to something that is morally wrong or unpleasant. For example, you could say "Es una situación fea" to mean "It's an ugly situation."