Pastel Punk is a term used to describe someone who wears stereotypical punk things (chains, spikes, combat boots, etc.) but in soft pastel colors. They often dye their hair like regular punks as well, but opt for more pastel shades than electric dyes.
Pastel Punks often don't behave like regular punks, having more of a love for aesthetics and clothing than a 'fuck society, let's all stand up against the government' attitude. They're essentially the uwu version of punk.
Pastel Punk is incredibly similar to Pastel Goth in that they both do the same thing: take small bits of the original culture (punk or goth) and mix it with their own culture (pastel). Both of them are not really about the music so much as about the aesthetic.