In fact, not only the Luigi's Mansion theme, but also arrangements of music originally from Super Mario Bros 3 and Super Mario Land can be played in the Luigi's Mansion stage for the Super Smash Bros series including Wii U and Ultimate.
Is it? Obviously you could say no because it's not scary, but scariness is kinda subjective. LM1 has some pretty creepy stuff for a Nintendo game, which could be pretty scary for a little kid.
BTW I don't actually think it is one. I just saw some guy on GameFAQs say it was, and I thought the...
I wouldn't be surprised to see her given one spinoff playable appearance just to promote Luigi's Mansion 3 (happened to Honey Queen & Princess Sprixie in respective period), but does she has the chance for any further participation? And is she objectively a good addition? Her advantages might...