Here's an example:
Mary had never concealed Marcos as though he were an (ugly) scar. No, far from that. She'd always exhibited him around like a sexy birthmark.
I think the word ugly is unnecessary here, since a scar is assumed to be ugly. A birthmark, on the other hand could be either ugly or beautiful.
The only reason to use ugly, in my opinion, would be to keep things symmetrical (ugly scar, sexy birthmark).
What the best option in these cases? Symmetry or simplicity?