Mariane is daddy's little girl. She loves her guy, Valère, there's no doubt about it, but she's used to saying yes to everything her father Orgon asks. So, as you might expect, she's a little confused when her father asks her to break off her engagement with Valère and marry Tartuffe. Only with the help of Dorine can she avoid a lifetime of unhappiness. Indeed, Act 1, Scenes 1 and 2 tell the whole story. In the first scene, Mariane can barely get a word out of her mouth when Orgon tells her that she's to marry Tartuffe. In the second, she doesn't have to get a word out: Dorine does all the talking. Literally. Mariane doesn't have a single line.
This isn't to say Mariane is a weakling or someone worthy of disrespect: she's really just an inexperienced girl whose father has gone off the deep end. You can't blame her for trusting her father – who has, as far as we know, helped her make good decisions for most of her life. Mariane is a victim, and with a little help from Dorine, she comes to see the truth.