A bumper crop is more of a boon than a bane to farmers. It can be either, but, depending on the market situations and the business opportunity in certain cases.
In most cases, a bumper crop is a boon to the farmers or a blessing. They grow more increase the income from sales when farmers have a bumper crop. Even so, this depends on demand availability, transportation availability and/or storage facility availability. When the goods of farmers reach the market in a timely manner, they are more likely to earn more returns, and it becomes a major success.
Assume if you had a bumper crop but many farmers growing the same crop as users, that would indicate you are not the only person with plenty. If your goods reach the stores first or soon enough, the only chance you might be able to find a great price from the market. Most farmers often do not prepare well enough and their bumper harvest ends up as a failure caused by lack of storage and transportation.