As I said earlier, "This is [...] consistent with Dora's unpredictable jealousy."
The emphasis is on "unpredictable." Sometimes she goes into a jealous funk, as when Marten mentioned offhand that he still sometimes thinks about Faye and even girls he crushed on in high school and she said, "You can go see [the movie] yourself. Or take one of your four other girlfriends."
Sometimes she has a minor psychotic episode, as when she menaced Cosette with a scary grin and a sword for daring to ask Marten out.
And sometimes she couldn't care less, as when Hanners charged into CoD all guilty over having invited Marten to poke her, and Dora laughed it off and poked her in turn.
Now I wish to emphasize I'm not one of the "OMG Dora's a c***t, dump her bitch ass, Marten" crowd, so no need to reach for the dick-broom. However, I do think it's an issue Dora needs to work on, because no one should have to go through a relationship walking on eggshells, uncertain how their partner will react to something at any given moment. If Dora predictably and consistently went into a jealous funk whenever another woman showed chaste affection to, or unreciprocated interest in, her boyfriend then, assuming he could live with that, Marten would know not to touch or be touched by other women, nor to mention old crushes in her presence. If Dora predictably and consistently threatened women with violence for so much as viewing him through peripheral vision, Marten would know to break it off with her ASAP, change his front door lock, and maybe get replacement ATM and credit cards just to be safe. But as it is, her behaviour is sufficiently unpredictable that if it keeps up--and personality flaws don't just go away by themselves--he's going to be more and more at a loss as to how to handle it. So Marten, sooner rather than later, needs to impress upon Dora that she needs to do something about it, for the good of them both.