It's one of those weird things in music where an artist will just have this one massive hit and they're never heard from again a few years down the line.

Is it that the song become bigger than the artist? Or that they are considered a joke?

For instance Gaga, Britney and Katy could've easily been one hit wonders because their debut singles were huge, but the controversy surrounding their image managed to help them make headlines thus causing them to be bigger than the song.

But then there's artists like Cassie, Daniel Powter, and a few others that had huge hits and then are never heard from. Their follow up singles all flopped. Is it because they had a basic image?

I swear, if it wasn't for Hot N Cold, Katy would be a 1 hit wonder
if it wasn't for Poker Face, Lady Gaga would be a 1 hit wonder

now a days its either your a hit or you get dropped at major labels

In the case of Taylor Swift, smaller label like hers can mold her into success

In certain cases the song gets bigger than the artist, they could put out just as good music or even better and still be stuck as a one hit wonder.

People become one hit wonders because there is little interest in the performer. Only their single attracts any interest.

what I got from this is that order matters

So let's say they released those singles first and their debut singles second...would they still have been 1 hit wonders???

In Gaga's case, no. Her album, theme and videos were strong enough to continue her success.
To answer the OP, the song is bigger than the person, and they may not have a strong enough USP to last.