Fair-weather

[PussyCat]

A fan that only supports their team when it is winning, or only likes a team because it is popular or successful. Apt to jump on and off the bandwagon as their team has its ups and downs. They ultimately often do not care too much for the team.

[Tanya Shivari]

Made or done in pleasant weather, or in circumstances
involving but little exposure or sacrifice; as, a fair-weather voyage.