What is Unicorn Princess?
1.
Someone who is overly proud of small accomplishments.
A braggart, but generally a great person who just likes to boast.
Also, could be considered a slang term for something so outrageous, silly, and/or trivial all you can do is chant:
"UNIcorn PRIN-CESS!!!!" and raise the roof.
"Can you believe I got that working in-game before you did? I can, booyah! It's cause I'm a UNICORN PRINCESS!!!"
See