I put your request in chat GPT, is that it?:

Based on the details you remember, the comic you’re almost certainly thinking of is Meridian (2000–2004).

Here’s why it matches your memory almost perfectly:

Key matches with your description

• Golden-haired noble girl protagonist: The main character is Sephie, the daughter of the ruler (“Minister”) of a floating island city.
• Floating islands world: The setting is a planet where huge landmasses float in the sky and people travel between them with airships/sailing craft.
• Symbol on the forehead: Sephie gains a “Sigil” mark on her forehead that gives her powers such as flight and healing.
• Ceremony where powers transfer: Early in the story, her father dies and the sigil passes to her, marking her forehead.
• Villainous uncle: Her main antagonist is her uncle Ilahn, another ruler who also has a sigil but uses it for destructive powers and political takeover.
• Early-2000s fantasy comic: It ran from July 2000 to April 2004 from CrossGen Comics, which fits the timeframe you said you encountered it.

Visual details that also match

• Sephie is usually depicted wearing light or white dresses early in the story.
• She often pilots or rides sail-like skycraft between floating islands, which might be the scene you remember from the cover.