Are Habits unlocked by a dragon's stars?

Yes for unlocking the slot, no for leveling it. That's the important nuance:

• The Habit slot unlocks at a star tier. Locked screens say it plainly: "Reach N star(s) on [dragon]". Count on roughly one Habit every 2 ranks.
• But leveling the Habit (1→5) does NOT depend on stars: that's done with a Breedmark + Core from chests.

That's why a dragon can show plenty of stars but few leveled Habits: the slots are open, but you haven't yet spent the chest items to level them.

So two separate levers: Relics (stars) open the slots, chests (Breedmark/Core) fill them.