I would say yes, much too early, unless you have a team helping, you won't be able to beat everyone in mooshu at that level. I would suggest doing Grizzleheim and/or Wisteria to level yourself up. All side quests will help.
Level 28 might not be too early for Mooshu, it mostly depends on how experienced you are and whether you have adequate gear. If you start questing there and survive fine, then your level is fine. If you find yourself struggling, then you'll know you need to increase your levels some before going back to try again.
The exception to this general rule is for Storm. If you're Storm I'd recommend you should do side quests right now and level up to get a little more health and accuracy under your belt (from level 30 gear). In my experience, having taken every school through Mooshu solo, I had the roughest time with Storm.
No matter what your school, at level 30 you will be invited to Aquila for the Mt. Olympus dungeon. Take friends in, and be sure to run that dungeon at least once or twice. A great wand drops there - guaranteed! - that will give you +10% damage. Also, the Zeus hats, robes and boots that drop there are the best you can get until about level 60. It's a long dungeon and difficult, but well worth it.