It's a cartoon character so it doesn't really matter (cuz y'know, not real and all), but if it's important to you than simply envision the Fionna from the Fionna and Cake subseries where she's a working adult theoretically in her mid-20's to early 30's. She still adapts the same style as her otherworld counterpart a little later in the series, so it shouldn't be a problem.
If you do still have a problem with it, then imagine this is an adult actress dressed up as Fionna. Again, they aren't real, and you're welcome to input your own view on a fictional character as well as the creator's allowed to make it. But you shouldn't force your view onto others. You chose to be here. You can choose to leave.
TL;DR: Separate reality and fiction, please. It's better for you mentally in the long run.
(You also seem a bit too young to be here. Porn rots the brain, please be aware of its consequences and take responsibility for them when you're an adult. You're still growing, and it is dangerous if you're set solely on sexual pleasure before you gain a stable foothold in society. If you are under 18, you should not be here at all. And if you are 18: grow up, it's a drawing and you chose to be here.)