I did the quest "Retaking Thirsk", but apparently, her dispostion lowers if you kill:

I got her the 50 Riekling spears, and decided to show of my flashing Amulet of Mara. No marriage option.

Okay...*drops all armor*

"Can I take it?" Sure you can!

And still no marriage option.

Are there more ways to increase her disposition?

This makes Narfi very, very sad. No more sleep. No, no...No.

There are no other ways to increase her disposition as far as I know. If you kill [^ spoiler], she will have a lowered disposition towards you forever and therefore cannot marry her. Giving her the spears does not raise her disposition enough to make up for killing [^spoiler], because the disposition system in the game weighs killing an NPC's friends/relatives very heavily in the negative direction. I would suggest loading a previous save (if you have one) and completing the quest the opposite way, otherwise I'm almost certain you're out of luck.

I had to make a new character, luckily Solstheim can be accessed as soon as exiting Helgen.

The other character was level 12, though. So no big deal.

Quickly went after those spears, and she finally made a comment about my Amulet of Mara.

thanks for the reply, I had never thought the disposition system went that deep. I always thought that it affected an individual, and not relatives/friends. Especially during quests.