Armor is for protection, not sex appeal.
A live woman is a far better marriage prospect than a dead woman. The whole sexy armor troupe is about serving the male gaze, not doing anything for the woman who is fighting.
Armor is for actual fighting, not walking around.
It's heavy and doesn't breathe and you can't move very well in it. A realistic knight has a squire who's in charge of packing all that stuff and having it ready for battle. Certainly a knight in wartime would walk or ride with some level of protective clothing, but it wouldn't be full armor.
Most women don't have itsy bitsy bodies with enormous boobs.
The stereotype of women with tiny bodies and boobs that made them tip over is, again, created by men for men and is pretty rare in real life.
Real life actors or models that look like that are 1) chosen for those characteristics and are a small minority of women and 2) usually surgically enhanced. Larger breasts are more likely to show up on women who are more voluptuous and/or muscular overall. Like a shotputter. Even then, there is a mix of body types.
Drawings, anime, and 3D representations (like Barbie) that show tiny women with breasts the size of their heads range from almost to completely unrealistic. Most men and women I know roll their eyes at stuff like that. Real life women with natural breasts that huge suffer from horrible back pain and usually have reduction surgery.
Armor is generally custom fitted.
There is not one size and shape of armor for men, so there wouldn't be for women either. Men are different heights, have different torso to leg proportions, different chest sizes, etc. If you don't think most women would be tall enough for armor, consider that teenage boys (who were not done growing) often went to war.
If you can account for these differences, then it's not hard to also account for differences with female shaped bodies too. Even a very large-breasted woman probably doesn't have a larger chest circumference than many male warriors. A smaller waist to hip proportion should also be easy to deal with.
The stereotypical boob armor is not just stupid, it's dangerous.
It should be obvious to everyone that leaving gaping holes in armor to show off skin is counterproductive.
But even when the woman is covered, shaping it for breasts causes no end of problems.
Attractiveness is in the eye of the beholder.
Fighting well then looking good when she takes off her armor will catch many an eye. Fighting badly (which includes wearing useless or dangerous armor) is a turnoff in all cases. If a potential husband thinks a women shouldn't be a knight, then fighting badly will just reinforce his opinion and make him less inclined to be interested in her. If female knights are normal in that society, or at least if the potential husband is okay with it, then fighting well is an important part of the attraction.
Knowing how to choose and use your gear is essential to being a good fighter.
Besides, even male knights would have a few gems on the armor, head piece, shield, or sword. They might have coats of arms impressed on the shield. Armor might even be painted. So if you want creative clothing, here's your chance.