Titanic is a fun film. Whatever it lacks in substance, it makes up in abundance in style, silliness and breathless romance which is basically a lot to get out of a night at the movies.
I'm pretty fed up with people reducing quality cinema to cinematic art. Not every novel is War and Peace, some of it is Pride & Prejudice or The Decamerone.

Maybe that's a controversial opinion to have, but I'm willing to trade every single Bergman, Von Trier and Almodovar movie for one The Lord of the Rings or one Finding Nemo. Telling a simple but compelling story in such a way that people who normally like their entertainment in bitesized pieces are willing to sit down for hours on end to see it, isn't necessarily low brow either.