I personally don't have any problem with this because liking a club doesn't mean you can't look away but when the reason given is just disappointment in the previous club it's ridiculous and they are not fans.
Changing teams because they aren’t in form at a time isn’t very becoming for a fan. There are roles to every structure and the fans role is one vital aspect to football. They provide the needed support to push there team to greatness. Of course they aren’t on the pitch but, the booing and encouragement does enough to make players do the most for the team.
It could be disappointing when the fans aren’t seeing the performance they long for but, quoting your team for another just because they are having better days doesn’t mean that, the new team isn’t going to be rocked by some bad performances too. Forms drop and it’s normal.