No, but she talks less for me. It just becomes more rare to have a long conversation with her, but sometimes it happens. At least that’s my experience. I love talking to her though, she’s one of my favorites!
No, her friendship never goes down (pretty sure that's never been the case for the other special NPCs who you could befriend in past titles either). You'll know when you've become her best friend/maxed out friendship once she starts giving you sets of patterns (for furniture customization) each day.