It is if you know where to go after most of the markets close.
In Munich, there are 4 main destinations that I can identify that still have that "Christmas Market feeling" and yet stay open past 9 PM. My suggestion is to hit up one of the traditional markets first, and then decamp to one of these destinations for the rest of the evening:
1. The Pink Market - this is Munich's "gay market" and true to form, they stay open later! Not that much later though... 10 PM. It's a great place to stop by on a weeknight, when you don't really want to be out that late, but when 8-9 PM is just unfeasible. Also, there are nightly shows her, so it's an entertaining place to have a couple drinks.
2. The Stachus Ice Rink ("Münchner Eiszauber") - this small ice rink at Stachus/Karlsplatz is also open until 10 PM. There is a structure built in front of the rink with an upstairs viewing platform, complete with heating lamps, and a downstairs area behind the food and Glühwein kiosks. The ice rink is pretty much always crowded, and it can be pretty amusing watching people falling all over the ice after a drink or two.
3. Alpenwahn am Viktualienmarkt - Behind the Viktualienmarkt, and the Schrannenhalle, there is a tiny Christmas Market (the Alpenwahn Glühwein stand and a couple other little kiosks) that is open until 11 PM. I highly recommend this spot- it's cozy and is in a great part of town for a pre- and post- Glühwein stroll.
4. Tollwood - always a good fallback, Tollwood is the big winter festival on the Oktoberfest grounds. It's part Christmas Market and part concert/entertainment venue. Tollwood is open daily until 1 AM, and it runs past the Advent Season until New Years. Tollwood is a great place to buy gifts - there is an entire tent for vendors to sell all manner of gifty items, and there are food tents and a museum/exhibition tent. Outdoors, Tollwood hosts the usual Christmas food stalls, and also has a traditional Feuerzangenbowle (flaming mulled wine- a fun German holiday drink, if you've never tried it).