Two of my three children were thumb-suckers and both grew out of it by age 5 or 6. They do not have teeth problems or alignment concerns. In fact, each has a beautiful smile!
At first I worried about the thumb sucking and thought my babies used it as a cue for hunger. So you may want to check--is baby hungry?
If you really don't want her to use her thumb, then you could offer a pacifer (because it can be taken away), however, she may be like my kids and refuse the pacifer. (My babies spit it across the room.)
I quickly learned, if they want their thumb, they're going to get it!
In general, thumb sucking is not a problem until permanent teeth start erupting at about age 5 or 6. So it helps if the child quits sucking the thumb by that time. Most will without any intervention by you. Some kids need a little encouragement.
With my daughter, we helped her quit sucking her thumb during the winter after she turned 5. This way when she started kindergarten, she was done. She still has the jewelry box 'reward' we gave her for quitting.
With my son, he took a little longer to quit and was not motivated by rewards. Finally, he realized he wanted his 'big teeth' to look nice, so he quit on his own.