Um, um, um.... what? We have the fourth highest airspeed in the game (two of the people above us requiring special conditions) and five midair jumps, we're by far the most mobile character in the game in the air. How is this a thing.
....Ahem. Aside from that... being light isn't a good thing, but it's not as crippling as you might think. Our light weight and floatiness drastically reduce our survivability, but they also make it very difficult to combo us- things that work on other characters may not find as much success with us, and in fact this can help us to avoid kill setups that might plague other fighters. Rest having frame 1 invincibility means that we have a surefire way to escape strings, which helps this immensely.
Range is a real issue that Puff struggles with, however, and it handicaps us in a lot of matchups where character have more than us. We're not completely helpless, though, with moves like Bair and Fair that still cover some distance (both can also be safe on shield if your spacing is absolutely on point). Dtilt also has some, but it's not really a very good move other than this so its not that useful to note.
The beautiful thing about Puff is that at a glance, she does look terrible; she possesses many traits that in other games would be objectively horrid, and would easily hold her back. However, we also have a lot going for us in this game, such as fantastic aerial mobility (cough) and potent combos that usually end with one of the deadliest moves in the game. We're definitely not Shiek, for sure, but we're certainly not the trash-tier character people might make us out to be.