First, I'll acknowledge that "serious" is subjective, however there is (probably) consensus that sentences imposed are reflective of the seriousness of a crime.
USA TODAY examined 2,598 written judgments in border-crossing cases filed in federal courts along the border since mid-May. In nearly 70 percent of those cases, migrants pleaded guilty and immediately received a sentence of time served, meaning they would spend no additional time in jail. Another 13 percent were sentenced to unsupervised probation, including a condition that they not illegally re-enter the United States. In both cases, that meant they would immediately be returned to immigration officials to be processed for deportation, leaving them in essentially the same position as if they had not been prosecuted. full article here
So, was there any point (in 84% of these cases) to invest time and effort in a criminal referral and prosecution? Does that suggest that the judges don't think that a serious crime has been committed?
(Feel free to migrate to Politics if that seems to be the better place for this)