Music genres have a definition for a reason. It doesn't fucking matter that gothic as an english word has been used to describe a tribe, an architectural style or a genre of literature, in the context of music and youth subculture goth(ic) does describe a certain genre, period. You can talk about how narrow-minded that is, it won't change this fact. You can't just call an apple an orange, just because you like it better. For the same reason no one labels Mozart Hip Hop or Elvis' music as Techno. So yeah, that is what genres/labels serve: to help define musical styles and make it easier for people to find similar music.
Now I wouldn't say that gothic metal is even goth- as in gothic rock- (much how deathrock has got nothing to do with death metal and glam rock is not the same as glam metal). But Nightwish and similar bands are neither gothic rock nor -metal.
LOL, seriously, that's one ugly fucking cover indeed. No money to hire a good designer and apparently the music is trash too.
But what's generally the deal of gothic metal bands having cheesy and ugly covers?