Honestly I think the glass cups would be for aesthetic. Her office, home and outfits are simple, streamlined and modern. She may be posh, but Royal Albert tea set would be too fussy for her. Which is also probably why she only has black tea with no milk or sugar. Or, its probably for the simple reason of having milk and sugar adds more unnecessary steps to a scene and visually you can see the liquid. Meanwhile, there is no kettle in her office… It’s also a contrast to the fact Ted has a kettle and teabags at his flat and adds milk to his and Roy’s tea. He also has a traditional American-style coffee machine in his office yet you never see him use it- they are always getting takeaway coffee.