It's always hard to know what a poet or other writer truly means, but I do not see any signs of irony here.
Instead, I see some amount of a sign of a struggle. At first, she is thinking about all the things she has lost. She thinks about where she used to sit, where she used to lie. She thinks about how no visitor will ever sit under that roof again. So at that point she seems sad.
But then she gives herself a talking-to. She reminds herself that all on Earth is vanity and true treasures are stored up in heaven.
So I think she's serious because she is weak at first -- she's tempted to be unhappy, but then she snaps herself out of it. That sounds realistic to me.