When running ddd from terminal, warnings like these appear (in terminal):
Warning: Cannot convert string "-*-helvetica-medium-r-*-*-*-120-*-*-*-*-iso8859-*" to type FontStruct
(Annoyed? Try 'Edit->Preferences->General->Suppress X Warnings'!)
Warning: Cannot convert string "-*-helvetica-medium-r-*-*-*-100-*-*-*-*-iso8859-*" to type FontStruct
Warning: Cannot convert string "-*-lucidatypewriter-medium-r-*-*-*-120-*-*-*-*-iso8859-*" to type FontStruct
Warning: Cannot convert string "-*-lucidatypewriter-bold-r-*-*-*-120-*-*-*-*-iso8859-*" to type FontStruct
and fonts in ddd are rather ugly. This is easily fixed by installing xfonts-100dpi package. Please add xfonts-100dpi into ddd dependency list.
This problem is long known, and was reported on [1].
[1] https://stackoverflow.com/questions/32533838/warning-when-running-ddd-in-ubuntu