Yes, Marissa Mayer will be able to increase Yahoo's profits, because the stock in Yahoo has gone much higher in the last few days because of the news of the new public offering. She has already done a good job. Yahoo might have lost some ground in the United States, but it is still very popular in China.
No, Marissa Mayer will not be able to increase Yahoo's profits. Yahoo has not been popular for many years. What they have to offer in terms of email, web search and general social features has been greatly improved upon by Google. Google is now the largest and most trust internet content provider and I don't see anyone running back to Yahoo in the near future.
I don't think it matters who the CEO of Yahoo is, the company itself, the brand, is not going to ever catch up to Google. They can acquire tons of other companies that can boost their profits, but that is pennies in the bucket compared to the power that is Google. Google has shown itself to be the better search engine, and has an amazing ability to create great apps, content, and software. Their momentum is not going to be stopped by Yahoo!
Marissa Mayer will not increase profits exponentially. She has been CEO since 2012. Yahoo actually seems to be more spammy and unreliable lately, especially the e-mail part. Unless Ms. Mayer comes up with a revolutionary idea, Yahoo will probably continue its success, but not be the golden child it was in the late 90s again.