Janna is actually the exact opposite, and aggressive poke support. Starting shield will give you arguably the strongest level 1 trading ability in the game. You can use her tornado as either disengage or to ensure the trade goes in your favor by knocking them up the second they move to retaliate (this level of disruption on top of a shield is really strong for short trades). Using tornado when you and your ADC aren't in a position to follow up (so the tornado is all that hits) is a waste of mana and cooldowns. It used to be good, but the nerfs to Janna's damage made this subpar. Don't only W. Weave it between autos to trades unless you're using it to peel for a disengage. Max E first, W second. The ideal trade for Janna is Auto-> Shield - Start Q -Auto - Proc Q -Auto - W - Auto, but sometimes you'll have to cut it short which is Shield - Auto - W - Auto and saving tornado for when they try to engage. If your ADC is competent, you can give it to them right before they take damage to negate the damage and give them a good window for retaliation.
The biggest mistakes I see with Janna players is that they're either waaay too passive and don't open any opportunities for their ADC because they think sitting back and tossing out tornadoes from the bush is the best Janna can do, or they're way too ham and don't know when to back off. I started off as the former, transitioned into the latter, then found a happy medium and fell in love with Janna as a utility carry champion.
Yes, you can stop dashes and gap closers like Leona's E with a well timed tornado.
Hope this helps.