I really think some of you just don't know what crackship means
a crackship has nothing to build on within canon. You have to make it all up yourself. There is nothing in canon for you to interpret as ship material. It's all guessing and interpretation.
it does not mean "I think there is a better ship for this character" or "I don't think the interactions they have can be read as ship material" because both of those are very subjective.
If you have a base of any kind to build on it's just not a crackship
Ginny/Tom is not a crackship because it has the whole chamber of secrets situation to pull from.
Harry/Tom isn't because they are literally enemies so they interact over and over.
Stuff like Bellamort is not because she has that whole obsession thing going on
if a character has any sort of meaningful interaction with another character then you have something to work with and its not really a crackship
yeah there is some outside circumstances that can make characters seem less so like for example archetypes or things we just know would be true (stuff like being in the same house or having similar interests)
also being a crackship isn't even like a bad thing it's just another category of ship if you want to ship your two favorite characters just because they are your favorites then you should you can make any ship work if you try hard enough. It's not hard to find a connection somewhere.