If you have no children of your own, and your nieces give you the sacred task of bringing something to their grandfather's memorial, and you know all the baggage that comes along with the memorial for your brother, not to mention all the baggage you're carrying yourself, trust me, dollar store balloons are NOT cutting it.
She's not in a bad mood. It's more about control in an uncontrollable situation. She clearly has all this baggage from Jack's death, and it's spilling over into her feelings about William's death, and the only thing she can do to help Randall is get those balloons. We've seen that Kate is a great administrative assistant, so you'd best believe that those balloons are going to be perfect. She might have been snippy to someone undeservedly, but I think we've all been there in times of stress and grief.