It's not Tomfags, it's Jannafags.
Tomfags like Tom, and if you think about him as a character for even a second, his personality, his needs, you'll see that Janna is pretty much the worst possible gf option in the cast for him.
Meanwhile, Jannafags tend to see their waifu through rose-tinted glasses and imagine that she'd be the type to open up and reveal her (imaginary)"nice side" once you get to know her, not understanding that Janna's a multipurpose allaround plot device cardboard gag character whose enitre personality is "creepy smug weirdo, likes to see others suffer, amoral to the point of comedy, likes the occult".
For their headcanon Janna, a demon prince would be the perfect wish fulfilment bf because prince, demon, magic powers.
They don't care that canon Janna would grow tired of Tom and his issues in a couple of hours. Janna likes occult things, Tom's a demon, "it's perfect"