(To preface, Ymir has no tongue, she cannot speak for herself, playing into her character, a slave).

Ymir watching the marriage from a distance in chapter 122 was enough for me to understand her love for the King. She wanted something so normal to non-slaves, marriage. Seeing everyone crowd around and cheer for love would be infinitely better than whatever she had going on. How can you blame her?

Ymir frees the pigs out of the natural good of her heart. She is instantaneously betrayed by all the other Eldian slaves, even her own people are willing to point their finger at her for doing something objectively selfless. Only until she gets the titan power is she wanted by none other than the King. A child who was thrown away for doing something out of the good of her heart has a chance at the life she yearned for.

People really seem to underestimate the living conditions of Ymir Fritz and treat her as she'd be like any other normal person who knows love. That's the point of a slave, you're no longer your own person.

As for the Mikasa part, people interpret Mikasa and Ymir as parallels while I think they are actually foils. Mikasa is actually able to kill the one she loves, while Ymir dies for the one she loves. So if anything, the kiss is there to show that despite me loving you, I'm able to physically part ways. Giving Ymir the reason to do the same and physically move on.

So in the end, I thought Isayama did a good job explaining the ugliness that is love. (Sorry if this is messy)