I was going to save this for a DS9 season seven rant I've been planning , but I might as well get this out of the way now since we're on the subject of Vic Fontaine. Most of this is a subjective point anyway.
Here's why I think Vic Fontaine was a bad addition to the show:
I don't like Frank Sinatra-style jazz and swing music. Yes, I know Sinatra was a great singer and a musical icon, but I don't like his music or his music style. Sorry, but that's just me. And that's the problem with making music a big part of a show - music is very subjective. Imagine this: I'm a big fan of R.E.M. and if I'd had my way they would've had an R.E.M.-style band instead of Vic Fontaine. That would be great wouldn't it? Well, some people (heathens I call them, but that's just me) don't like R.E.M. so for them, interrupting the show to have Michael Stipe sing "Radio Free Europe" is just going to be an annoyance. That's how I felt every time Vic sang on the show. Half the time it felt like it was just in there to pad out episodes that ran short or save the writers having to think up new scenes.
None of this would be a problem if it was a one-off thing, or in the background. But that's not what happened. Not only was Vic Fontaine brought back multiple times - he was the focal point of several episodes. And Vic as a character isn't all that great either because he's just an archetype. "It's Only a Paper Moon" was a great episode, but that was more down to its focus on Nog's character rather than having anything to do with Vic. They could easily have done that episode without him.
Also, I felt like Vic was just a result of the writers' self-indulgence and storytelling excesses gone too far. It was seriously cringeworthy when he pulled Sisko up on stage at the end of "Badda-Bing, Badda-Bang" just so they could have Avery Brooks sing a duet with James Darren. The fact the song they picked was "The Best is Yet to Come", preceding the absolutely craptastic "Final Chapter" story arc just made things worse and reeked of arrogance and overconfidence.
So yeah, not a Vic fan.