zodiackelly laughing is, for many of us, a totally involuntary action! we laugh when we find something funny but also, sometimes, we laugh when something else happens, a kind of nervous laugh. My mum used to laugh when I fell of my bike etc (much to my annoyance), now I laugh much to my dd's annoyance) when the kids hurt themselves.
It's a kind of nervous response and does not really equate at all to how one feels.
I wonder if she (possibly as a new member of the group) was nervous.
So in answer to your question... "Still, there may have been a more polite way to go about it, no?"
No, not if it was a nervous reaction. But she could have apologised for laughing.
Also, I am a bit of a butterfly brain and well established friends would not be offended if I drifted off topic about stuff. But I am 50 and have known my friends a long time!
I remember in my very early twenties meeting a new friend and early on being very put off by the fact she would suddenly say in the middle of a conversation "Oh, I really love that scarf!"
I suggest you just keep going to a group you like, either avoid her or engage with her, if you wish to, keep off topics that you yourself feel may be a bit 'out there' too early on ....and forgive her for her slight over reaction to your comment and she will forgive any flitting off topic from you! 
Come back and tell us in a year when you are best buds! 