It sounds masculine in Spanish because it is a masculine adjective. The fact that most masculine words (especially adjectives) in Spanish end in -o doesn't make it any better.
And I agree, it's a strange choice. If they wanted a colourful name for a girl they could have chosen
Iris. Or even
Celeste! :/
“Dear optimist, pessimist, and realist – while you guys were busy arguing about the glass of wine, I drank it! Sincerely, the opportunist!”
-Lori Greiner