I'm looking for a word for somebody who supports things fleetingly and whilst they are popular, for example a sports team on a winning streak, or something which is currently in fashion.
For a sports team, bandwagon fan is probably the most common term in the US. Bandwagoner would be the more general, non-sports case, and most everyone would know what you mean although it's not very a common term.
If we're focused on sports, fair-weather fan also works well. It does have a slightly different connotation than bandwagon fan, in my opinion. A bandwagon fan is more likely to follow any team that is currently popular. A fair-weather fan is more likely to follow only one team (in a particular sport), but is likely to only act like a fan when that team is doing well.
"Dan is a bandwagon fan. He used to be a Yankees fan, but now he says he's a Red Sox fan."
"I never saw Dan wear a Red Sox jersey even when he lived in Boston, but now that they've won a World Series he wears them all the time. He's such a fair-weather fan."
It would be called a faddist.
A fad is any form of behavior that develops among a large population and is collectively followed enthusiastically for a period of time, generally as a result of the behavior being perceived as popular by one's peers or being deemed "cool" by social media.
Though the term trend may be used interchangeably with fad, a fad is generally considered a quick and short behavior whereas a trend is considered to be a behavior that evolves into a relatively permanent change
Times change, and old words go out of fashion, forgotten or replaced by new metaphors. For example, in the eighties, we'd call someone a trendie if they were a fickle follower of fashion. But forty years earlier, an airman might call someone a windsock; whilst his farmer grandfather would call the same person a weathercock. In both cases, they meant that the subject of their scorn would change as often as the wind, and might even end up facing in the exact opposite direction to that of an hour earlier!
Akin to the fair-weather friends mentioned earlier, our swimming coach used to deride what he called warm-water warriors. He suspected our dedication if we weren't prepared to break the ice - literally!
But what's the currently fashionable term for a fickle faddist? I have no idea, and besides, I'm not even sure I'd understand whatever new metaphor it uses. So even if I wanted to be fashionable, I probably couldn't hack it ... :-(
Which all leaves me with a few rather pedestrian options. (In case you missed the metaphor, I'm driving my car, of course. While texting? No, I don't have a death wish!) They include nouns:
- swing-voter (for a special situation)
- wishy-washy (very slangy, but often on our lips)
- inconstant (as the poet had it)
- uncommitted (a mild reproach)
- disloyal (a far stronger reproach)
among others. So far I've avoided mentioning some harsher terms, such as sycophant, and another vulgar word (sadly, the commonest in my hearing) that alliterates with its first syllable.
Not all these terms need be pejorative! As a pop song once claimed: "He's just a dedicated follower of fashion". But I suspect the writer of irony.
A fair-weather friend isn't too far off but the definitions all use it to explicitly mean a friend, not a supporter of a football club or any sort of team. Dictionary.com Fair-Weather friend
If you do a search for glory hunter or glory supporter you'll find most of the results relate to sports supporters who support a team or person that is doing, or is likely to do, very well.
front-runner is a term often used for the person who support the currently trendy winning team.