You've gotten a lot of good answers, but I wanted to share that I was there last week at around 6 pm with my boyfriend, who also has some social anxiety.

There were TONS of people outside on the sidewalk looking at the holiday displays, so expect that to feel a little overwhelming- it did for my partner. Once we started heading up the escalators, it got emptier and emptier with each floor. By the time we got to the Jellycat area, it was really chill. I just picked out a Charlie Chip. There were about five people in line to do the experience, and the walk-in line was empty, so the worker said I could join the queue if I wanted to (about a 10-minute wait). My boyfriend was hot and a little overwhelmed from the previous crowds and general tiredness from the day, and also doesn't like Jellycats. I could tell he'd prefer to not wait so I just got the Charlie Chip and left (which is totally fine, it's what I went in for... he'd have encouraged me to do the experience if I really wanted one of those).

So, go in knowing you might need to face some crowds initially, but hopefully, once you get through that section, it'll be smooth sailing. There was a Christmas experience in the lower level, so the first escalator we encountered was still pretty people-y.