is it bad that i ONLY use Lux?

No, Lux is the bomb.

Edit: You should still learn a second, non-Lux champion for when Lux is picked away from you or banned. Also you should know all the roles. But nothing wrong with having a main.

I mean if you're doing it at the expense of your team's overall composition, yes.

There is NOTHING wrong with that. If you can flex pick her like taking her mid or even support it makes her an even better choice for you. There is a player called Annie Bot and he almost EXCLUSIVELY plays . He has about 1 million mastery on her (since its implantation and not including previous experience) making him an amazing Annie player. One trick ponies exist but they are able to flex where they can take their champion like no other player can.

Side note: You should probably pick up 2 other champions you can fall back on in case doesn't work well with your teams setup. Even the player Annie Bot probably has some fallback champion in case his Annie gets banned or she just isn't a good pick for the team.

nah

tbh I think you're cancer and hate Lux's safe game style so very much

Well first it's ok to play this noob 0 skill champ,but you should learn one or two incase she's banned or taken

nope its alright to only know how to play 1 champ.

Idk, is it bad that I only use Lulu and go only botlane?

To answer your question, no. Absolutely nothing wrong with playing one champ. Just don't be an asshole about it, especially if you can't pick Lux for whatever reason.