Mage+Dancer Team is viable in this meta?

For attacking, sure. For defending, not that good. For example, a single Kagero with vantage can take out this team on her own. Or, a Hector pair with a good blue unit that can take out Tharja. Unless you are talking about the new defending mode, which I hadn't look into and am unsure about.

Hell yea that shits scary! Ranged teams with a dancer are bit harder to bait in my opinion. On Certain maps they can be a nightmare.

What @HECTORSMASH have said it's true.
Yesterday I have found a team that was composite of tharja,nino,bunny lucina with blade tomes,and a azura.
Wait a minute...Was THAT you!?
It was a pretty hard fight, only my Julia had survived,the battle was on that map that your team start on the left side & the opponent on the right side,and you have to break the wall to progress.
Man, without preparation,your team will make some people really salty.
But you had build a pretty good defense team.Congrats.

Yes and No, they are dangerous but players who know how to bait will easily 1 hit kill your team.

You should test their A.I. interaction by setting them off in Auto-mode in the 10th tower and see how the computer controls them. Rally attack, drawback, Re-position will help prevent them from stupidly going straight for a target and get one-shoted when they over reach.