No, the Sheridans should not have called off the garden party at the very last minute because someone they barely knew died. When Jose, Laura's older sister, says that Laura is being sentimental, she speaks the truth.
Sentimentality—sometimes called bathos—is false emotion, and this is what Laura is experiencing. She knows nothing about the working classes and is still feeding on a glow she experienced earlier in the day when she interacted with some workers briefly over party preparations. She decides, completely out of her head, that the the social classes are alike. This makes her feel good. When the man dies, Laura wants, based on false and overwrought feelings, to make a dramatic gesture of solidarity that affirms the humanity of the working classes—a solidarity she wants to feel but ultimately doesn't.
Of course, in reality, people don't cancel parties when people they don't know die, unless the circumstances are extraordinary. Laura, of course, is ironically showing the class difference between upper and lower (or "middle" and "working") classes in even suggesting such an extravagant gesture, as this would cause the Sheridans to throw away all the money they are spending on the party on a whim. This is something the working classes couldn't dream of doing—or expect—as they soldier on through their lives.
The complication is that class issues are glaring and do need to be addressed. The daughter is right that the working class in her culture is dehumanized and shouldn't be. It's wrong that Laura's Sheridan family can spend more on a hat than their workers probably earn in a several weeks of hard labor or that the workers live in hovels while the Sheridans carelessly throw extravagant parties with fancy pastries and hothouse flowers. However, fixing this situation calls for systemic social and economic changes that Laura can't begin to truly imagine or contemplate. As her preference for a pretty hat to solidarity with the poor shows, she's not willing to do what might need to be done—give up some small part of her own privilege and pleasure—so that the working classes can have better lives.