No, that's not ironic, that's a perfectly normal reaction to a person being rude. It's pretty insensitive to say or suggest your girlfriend's interests are just weird or stupid to you. If she told you that your collection or interests were weird, you'd be pretty miffed, too. Telling someone to grow up is a surefire way to hurt their feelings, too. No one wants their significant-other to tell them they're being immature, just for liking things
you don't.
Many women, including myself, are very interested in skulls, dragons, gargoyles and other such things. Whether or not you can fathom that is irrelevant. If you care for your girlfriend, you should be supportive of her interests, instead of thinking it's all fine if you mock her.
I'm sorry you're going through a hard time, trying to deal with your grief. The best thing for you to do is sit down with your girlfriend and have a talk with her. Explain how you're feeling. About your loss, about the house, about anything that's bothering you. Make sure you talk about how YOU feel. Don't accuse her of things. People don't listen if you start saying, "you do this", "you said that"; rather, you need to say, "I feel", "I think", make it an expression about yourself.
You say you wish she would understand. Have you tried explaining how you feel without insulting her? Have you told your girlfriend that you want her to keep the things you don't like (ie, the skulls, etc) in her own space? But especially, have you told her this WITHOUT saying they're immature and ridiculous? No one is going to be understanding if you're just seen to be attacking them. Neither can she read your mind. She's also dealing with your grief (and perhaps her own), because she cares about you. It hurts when someone you love is hurting.
If your girlfriend presents reasons she wants her dragons and other things in your house, try to be open and listen. If she's living with you, she has a right to add some decorations of her own. If she just stays the night, then you can explain, very politely and calmly, that you need the sense of security that controlling "your house" gives, during this difficult time.
Most importantly, keep communication open. Be understanding yourself. Tell her how you feel (because she won't know unless you do). Tell her, nicely, what you want or need. Listen when she expresses herself in return.
...And learn to think before you speak (since you said that yourself). Being a guy doesn't absolve you of that duty in a relationship.
