She doesn't fight like a normal character. Her moves don't operate like most charcaters' (for example, she doesn't get knockdown on standing normals outside of counterhit) and for the most part have always been bad for neutral. This leads to her Drive. Noel's drive is her best tool, which leads it to be used more than anything else by a typical Noel player, even some of the best be mashin' D awl dei.
Because Noel'd Drive have invulnerable frames, they can be both amazing reversals, or more commonly, a somewhat safe approach to mashing. This is something that makes people hate Noel, she has invincible drives that require no real timing or confirmation abilities and lead to average to great damage.
4D and 2D are probably the biggest causes for salt when someone fights Noel.
There's nothing cheap about Noel since everything she does can easily be dealt with, but if people don't know how to stop her, it becomes exceptionally aggravating having your hard work shat on by drive mashing. Not to mention that the greater majority of Noel's combos are very easy to perform, even her SMP combo takes no execution.
There are lots of reasons why people hate Noel, but it tends to boil down to drive is dumb, mash drive, no skill.