There's not sure definition of Hardcore but there are some characteristics that have been fairly constant with games that are called "Hardcore."
1.) Bloom, along with grey and brown.
2.) HD Graphics
3.) Some themes that involves killing or finishing objectives violently
4.) Dark themes and other things that people generally relate with "cool"
5.) High-production values
6.) some degree of challenge or opposition
7.) High quality (reviewed with 8.5 or higher)
8.) deep, sometimes overcomplicated, gameplay and stories
Pixel Art can be fun.