I'm one of those Christians who don't personally celebrate Halloween as you see it done in the US or as its taken off in most of the UK. I do some years go to an All Souls Service which is held at our church, which is for remembering those who have died. And having lost members of my family and friends over the years, I do sometimes go to that and pray for them.
The type of Halloween that is mostly celebrated now doesn't have anything to do with All Souls, but is linked to the old pre Christian Celtic religions, so I just don't celebrate that, and my children have been brought up not celebrating it. I know some Christians do go along with it, but we personally don't.
When I was growing up in Scotland (a long time ago!), I do remember All Hallow's Eve parties for the kids with dunking for apples, and some pumpkin carving and candles. In our village where it was done the pumpkin lanterns were put out to ward off witches and evil. I don't remember it being such a big thing back then with dressing up and there was no trick or treating etc the way it is now.
I don't celebrate Halloween for a number of reasons. I don't like the way the Trick or Treat thing has escalated in recent years. I work with elderly people, and know alot of them who live alone are pretty scared at this time of the year and just do not come to the door after dark. While alot of parents are very responsible escorting their children and only going to pre-arranged houses, its also not unknown for older children and teenagers to go out in groups by themselves and tend to do more Trick than Treat.
From a more personal point of view to me it feels to be a pagan festival so as a Christian I would feel uncomfortable doing that myself. Also having grown up in Scotland with the whole thing of pumpkin lights warding off witches and dunking for apples, well that just makes me think of the whole persecution of the Witch Hunting era and all the poor women and girls who died by either being burnt at the stake or thrown or ducked in ponds to see if they would sink or float. So to me personally, I would feel almost as if I were besmirching their memory by celebrating something, that when I was growing up, was based on the persecution and murder of others.
But, I realise that alot of people don't see it that way and view it as harmless fun. If people want to celebrate Halloween, I think that's perfectly fine, and I have no problem with them doing so nor would I be offended by that. In the same way I wouldn't have any problem with a Hindu celebrating Diwali, a Muslim celebrating Eid, a belief in crystal healing or an Atheist saying God doesn't exist. I would support and protest their right to practice what they believe. I just wouldn't be joining in with it myself.
I would hope that other people would understand that and not mock or belittle me for my personal beliefs, in the same way I would not mock or belittle them for theirs. So I wouldn't have any problem sharing a flat with OP's decorations. They are her decorations and yes they are on the outside of her door, but to me its her door and her right to put on it what she wants.
I do think that if its a house or flat share you need to take into account what other people think, and sometimes that means compromise. So for Christmas decorations etc like a tree - it has to be a case of everyone agreeing with what can and can't be done in the shared space.
I would suggest the OP drops her a note inviting her to have a talk over coffee where they can listen to each other and resolve it amicably.
Could I just point out one last thing to think about. Whenever Christians in particular, say anything, there are alot of people who instantly jump on the band wagon to belittle and mock them, and I would politely ask those who have expressed some of those sentiments to consider what I'm saying. There is a big move in the wider secular culture that expressions of religious belief should be confined to places of worship or kept privately at home. Christians (and possibly other faiths but I can only express about my own) are constantly being belittled, pressurized and told that 'Society has moved on, there's no place for your beliefs in public, keep it at home etc etc and not to comment on things where you might offend someone else by having a religious view on it.' It can feel as though its a constant persecution and pressure to suppress your beliefs and who you are as a person.
I'm mentioning this because the flat to the OP's flatmate, is her home and her private space. If the OP's flatmate is not even allowed to have any thoughts or expressions of belief at home then where is she?? And what does that say about us as a society?
I realise that the flat is a shared space, a shared home so its a slightly different scenario, but that's why the best way to solve any problems is to talk about them, get an understanding of how each person feels and reach a compromise.