Riding in nice office/casual clothes? I'm a little new to bike riding, I've only been a commuter for the past couple of years so maybe I'm only now observant of this sort of thing. So over the past few days I've noticed people commuting in what looks like stuff you would wear to the office. I literally saw a guy riding his bike in a shirt and tie today!

Granted it's still not too hot out, I live in Manitoba and lately it's been around 20-25, but has it suddenly become a thing to look like a fashion model while commuting to work or are the Humans around me just denim-wearing demigods who can't break a sweat? As for me, I'm usually wearing crappy t-shirt and a thin pair of shorts and I still sweat like a pig on my 8km commute to work. 

I guess what I'm trying to ask is whether or not this is normal behavior, maybe I'm just out of shape?
Those people you mentioned seeing in 'office clothing' most likely don't have a very long commute. Depending on where you live it may be like here in Switzerland; it has become some kind of trend. People show off their dutch style bicycles and feel awfully cool, which might explain why you perceive them as looking like fashion models. You will also notice how many young and trendy guys ride a women's bike because they idealize the authentic dutch hippie bike. I just pass by and smile at their confusion regarding bicycles.

For my part I ride ~ 10km to work on my old Batavus Champion, in every season. A little perspiration is normal, but I don't really need to change clothes when I arrive. 

I think the key to successful commuting over longer distance is having an exceptionally good deodorant.