Not really. Only a little bit of K. Rool support left, with a lot coming back around, and plenty of people complaining about the Mii costume. So, definitely not the case at all, if anything it made (most) K. Rool fans even bolder. The only support that left were the people who casually supported him because they thought he was inevitable (namely, whenever a character looks "likely", a whole bunch of people start supporting that character because they want to be on the "in" thing at that time).
Very, very few people stopped supporting K. Rool, and even some of the hardcore K. Rool fans who don't think he's happening anymore are still complaining about it, which is still raising K. Rool's attention. I don't see anything much changing for Isaac were he added. Again, only a few would stop supporting, along with any people who thought he was "likely" (there's far less people who think Isaac is "likely" than they did for K. Rool).