Gremlin and maybe cryptid are most likely being used in an endearing way. Kinda like "Wow, i'm so silly!" or "I'm so weird" respectively. Not necessarily negative. I see it as highlighting their faults as making them sepcial/'rare'. That's what i Think right now anyway.

I have friends that I refer to as gremlins in an endearing way, simply because they have chaotic energy. They never mind the label, and seem to embrace it. It’s just become a fun/cute nickname at this point.

People can call themself whatever they want, I reckon :)
Though gremlin to me is not very endearing. It feels grimy and dirty lol