Mayor Pete got elected Mayor because his campaign was essentially "I got good grades in college, I joined the military and I can form clear sentences." His presidential campaign is the same, but with "and I'm gay" added to the mix. As Mayor of South Bend, he accomplished nothing, other than adding green bike lanes and making one section of downtown look nicer. The town still sucks and the crime rate is still high. South Bend is the closest city to me, but I drive a longer distance to shop in other towns to avoid being shot, as there are often several shootings per week in the area where I would shop. Notre Dame even got its own zip code (the school is now in the town of Notre Dame, IN) to avoid being associated with South Bend.
On the whole, Mayor Pete hasn't done anything "bad" as Mayor. He just hasn't done anything. As Albion said, his resume is paper-thin - not just his political resume, but his life resume - and I don't get why he is taken seriously as a presidential candidate. Lots of people got good grades in college and joined the military, and anyone can keep a seat warm for a couple mayoral terms.
That said, I also don't see why most presidential candidates are taken seriously as presidential candidates, so don't think I have some kind of special disdain for Mayor Pete.