1. Poor acquisitions
Mayer's mergers and acquisition strategy at Yahoo is basically a graveyard at this point. It may not necessarily have been the wrong strategy for Yahoo (just look at how Instagram has worked out for Facebook) but her execution proved disastrous.
Mayer made 53 acquisitions during her time at the company, with at least 41 being subsequently shut down (anyone remember Bread?).
The worst of the bunch though appears to be Tumblr, which Yahoo paid $1.1 billion in 2013 and admitted in 2016 that it had lost $230 million in value since.