When she proverbially shat all over Cirnos' very hard-earned victory over her in Fairy Wars by saying "Oh, it's not like I would have ever gone all-out on a mere fairy now" within full earshot of Cirno and her friends.
Instead of Cirno's victory finally earning her and her friends some respect, everyone just blew it off and instead saw it as just an excuse to rib Marisa for a while; "LOL! You played around too much and that stupid fairy landed a few hits! Silly Marisa!"
Imagine how Cirno felt having her all of her efforts spat on like that? How she felt as people ignored that victory and continued to mock not only her, but also her friends, calling them things like the 'bakatet' or '⑨-tet' for associating with "that stupid fairy that thinks she's strong"? This whole thing was about Cirno and friends trying to earn some respect, because even the greatest efforts of their strongest member were summarily ignored before.
Not to mention Keine just mentioned all of that outloud to Yuuka to convince her to fight seriously, so now Marisa understands just how much she hurt Cirno's feelings just to try and save face.
