Not a dumb question at all...
And for all you immature, emo-hating, close-minded, judgemental, lizard-lipped, delinquents, listen up. SHUT UP!
Thumb holes are becoming fashion standard and many major labels are using them, especially in ski line clothing. If you don't like them, role the cuffs up and deal with it.
It makes perfect sense to want to give your hoodie thumbholes. It gives a unique style and comfort to the hoodie that makes it your own. Especially in colder environments. Allows you to put your sleeves into your gloves comfortably to transfer warmth and keep your hands warm. Perfect for cycling, skiing, snowboarding, skating, or just walking on your commute. I had them put on my hoodies and on my jacket. Very helpful when the temperature drops into the Teens and single digits with wind chills in the negatives (Basic Chicago winter).
Advice for doing your own, unless you are handy with needle and thread, don't try to do it yourself. Take the hoodie to your cleaners that can do alterations. Make sure you ask if they know how to make thumb holes, also you should be sure the hoodie sleeves go almost past your pinky finger. This gives you the extra coverage and won't pull the seams at the shoulders. Have them do the cut in the Cuff, below the seam. Make sure you line up where your thumb will come through on the cuff. If you have too, cut on a old hoodie or long sleeve T-shirt to figure out where your thumb fits properly so you can mark it on the cuff you want to use for thumb holes.