That depends on what you call “pretty.” Certainly, flowers don’t smell pretty to themselves; their aroma is intended only to attract insects that can help pollinate the plant. Insects, too, probably don’t find that a flower smells “pretty”; they simply visit the flower for its nectar.
One flower whose aroma is definitely not pretty is the carrion flower of Africa. The red leaves of this plant look just like rotting meat, and the flower smells like rotting meat as well.
The smell tricks certain flies that normally lay their eggs in rotting meat to lay their eggs in the carrion flower instead. The visits from these flies help pollinate the plant.
