How about other days of the week?
How crowded are you talking about about? Every seat taken? STANDEES? :excl:
IME Sunday afternoons tend to be extremely busy with weekenders/families/college students, etc. A mid-week departure is likely to be a relatively lighter load, but I'll confess to not having any specific data on that.
Since the Cascades are reserved, there should theoretically be no standees. I've never witnessed that myself.
When leaving Seattle it's unlikely that every seat would be taken, as there will be seats reserved for people boarding in Tukwila and Tacoma. By the time you get to Olympia, it's likely that there are more people exiting the train at each station than entering, so even a packed train leaving Tacoma will thin out somewhat. However, people headed for smaller stations tend to be grouped together, so if you're going all the way to PDX you may not notice any lightening of the load.
As the seats are generally wide and comfortable, it's not so much the crowded conditions ON THE TRAIN that make things unpleasant...it's the potentially long line in the SEA station for coach passengers, since unlike most Amtrak routes, the Cascades assigns seats in the station (at major stations...at smaller stations you simply get on and find a seat)