not bad taste in itself, no more than using Aerosmith or Maroon 5 music in an ad. 

If standing out and making a memorable impression is the goal of a performance ad like the one described here, then it's possible that even a bad performance can do the job of creating an impression. The question is - will the lasting impression also lead to sales?

To a large degree, it's all about name recognition.  If there is ample opportunity for the consumer to hear and/or see the name of the company, then it's a successful ad.  I'm not sure why rap would be any better or worse than opera or rock and roll.  Some people like one music genre and dislike another.  So, any time music is used, you are going to gain some consumers and lose some others.  If the business owner is aiming toward a very youthful demographic, it makes sense to me to use rap. 

If the ad gains the attention of potential consumers, then I do not see a problem with this type of commercial. In fact, the commercials which are "bad" are sometimes the most appealing. An advertiser's job is to gain the attention of the consumer; and if the rap gains the attention of prospective customers, then job done.

I don't think that it would be a big deal.  Businesses like that tend to have ads that are pretty odd anyway.  It would not be very strange because the owners of such businesses typically try to create gimmicks that will make them memorable.  So I see no problem with that.

If its his business he can do whatever he wants.  I'd be more curious about the quality of the rap, than the fact that he was rapping.  Additionally, there are a lot of different types of furniture stores out there.  If rapping will help him reach his target audience best, then I'm all for it!