Tomatoes are called "ugly" when they have physical characteristics that make them look less than ideal, such as being irregularly shaped, having cracks, or having odd coloration. These characteristics can be caused by a number of factors, including:

Temperature: Tomatoes grow best in temperatures above the mid-60s, and lower temperatures can cause ridged fruit and catfacing

Heirloom cultivars: These tomatoes are more likely to have physical defects like catfaces and sutures, but these defects are often only skin deep and don't affect the flavor