It's not that she's failing at trying to be Diane Kruger, it's that she is a young girl (she started dressing in stripper gear at 16, and she might be almost 18 now but that doesn't make her emotionally or sexually mature) using her body in an incredibly adult way, which is disturbing. If it were just the makeup or just the shoes or just the occasional lingerie piece worn with pants or even any kind of clothing at all, etc, that would be one thing. But she's out selling, promoting, and performing her music dressed literally as a stripper.
It's completely normal for teenagers to experiment with style, with sex, to want to shock others, and goth and punk are legitimate styles, yes. But she is not a normal teenager - she doesn't appear to have parents who actually parent, she's the object of media attention, and she's on stage presenting herself for consumption. All of that is a legitimate subject for criticism.
And she's not a train wreck, which makes her a guilt free object of snark. Other than the disturbing affinity for lingerie as clothing, the flashing, etc, she seems to have her act together. Which she deserves props for, but I can still hate on that awful effing makeup.