Tumblr founder David Karp and Yahoo CEO Marissa Mayer. AP Photo/Frank Franklin II In the wake of Yahoo writing down another $482 million from Tumblr on Monday, New York University marketing professor Scott Galloway had some harsh words for CEO Marissa Mayer and her decision to buy the social-media site for $1.1 billion.

Monday's write-down was the second impairment charge recorded on Tumblr this year. In February, Yahoo disclosed its first noncash impairment charge that cost $230 million.

In total, Tumblr has essentially lost more than a third of its value during the three years under Mayer's watch.

"Tumblr will likely go down as the worst acquisition in tech of this decade and summarize Mayer's tenure as CEO," Galloway told Business Insider.

Galloway lambasted the $1.1 billion deal as "overpriced" and compared Tumblr to a "porn site" for its high level of adult-site traffic. He narrowed down his thoughts to the following three points:

Yahoo overpaid from the start: Tumblr was reported to have lost $25 million on $13 million of revenue when Yahoo bought it for $1.1 billion in 2013. On top of that, neither Tumblr nor Yahoo has figured out a way to get Tumblr users to allow more ads on their blogs, Galloway argues. No tech IP value: Sometimes you can justify the price if there's a lot of tech IP involved, but Tumblr doesn't have much proprietary technology, Galloway says. "None here, just an aggregation of blogs," he says. Too much adult-site traffic: A quarter of Tumblr's inbound traffic was estimated to come from adult sites at the time of the acquisition. There's nothing wrong with that, but it hurts its advertising value, Galloway says. "Yahoo bought a porn site for $1.1 billion. P&G or Ford aren't going to take the risk(s) associated with advertising on Tumblr when they have better/safer options everywhere," he says.

All in all, Galloway argues that the bigger culprit falls on the board members who picked Mayer as CEO in the first place.

"The level of shareholder destruction here is borderline negligent. The board bears some responsibility as they are fiduciaries for shareholders and should have performed some elementary 'sanity' checks that would have produced red flags everywhere," he said.

Galloway, a marketing professor at NYU's Stern School of Business, is accustomed to being outspoken as the founder of the activist hedge fund Firebrand Partners. He has also founded two online businesses and is a frequent media pundit on the tech industry. Galloway noted he owns no Yahoo shares and is no longer involved with Firebrand Partners.

But he is also known for not being shy about speaking his mind. Last year he stirred controversy when he said in a Bloomberg interview that Mayer would be out of a job if she weren't pregnant. He was also responsible for a viral email in which he told one of his students to "get your s--- together."