As her wikia page explains:
Ten years prior to the creation of the FLDSMDFR she died of unknown
causes and her death affected both Flint and Tim heavily. After her
death Tim proved to have difficulties raising his son due to not
knowing how to properly show him the encouragement his mother used to
show him, however Flint would continue to be encouraged by the words
she spoke to him in his youth throughout his inventing carrier.
In the end credits, her ghost is seen haunting Patrick Patrickson.
So no, it isn't explained in the movie.
However, it is possible we might learn more in the upcoming television series. Here's what Wikipedia says about it:
On October 9, 2014, DHX Media announced that it will develop and
produce a television series based on the film franchise, titled Cloudy
with a Chance of Meatballs: The Series. The series will be
traditionally animated and will consist of 26 22-minute episodes.
It will take place before the first film, showing Flint Lockwood as a
high school student who dreams to become a serious scientist. In
his adventures, he will be joined by Sam Sparks, a new girl in town
and the school's "wannabe" reporter, along with Flint's dad Tim, Steve
the Monkey, Manny as the head of the school's audiovisual club, Earl
as a school gym teacher, Brent as a baby wear model, and mayor