First, let's see his complete statement:
(...) We won the evangelicals. We won with young. We won with old. We won with highly educated. We won with poorly educated. I love the poorly educated. We’re the smartest people, we’re the most loyal people, and you know what I’m happy about? Because I’ve been saying it for a long time. 46% were the Hispanics—46%, No. 1 one with Hispanics. I’m really happy about that.
Now, let's see some interpretations of what he said and them some explanations:
From this article, that criticizes who is mocking this phrase:
Of course Trump loves poorly educated voters. Who else would be asinine enough to buy his tripe?
According to this other, from Huffington Post (another article critical of the negative interpretation of his phrase, as a kind of prejudice against poorly educated people):
On the surface, Trump's pronouncement seems to confirm the worst fears of his critics: of course he loves poorly educated voters! Why else would anyone support someone whose policy platform preys upon those driven by ideology, a platform that cannot withstand even minimal scrutiny?
Perhaps it is because attacking the poorly educated has become a kind of American sport.
Marco Rubio used this phrase against Trump:
"That's really what Donald Trump is all about. He thinks we're fools."
In this another article,
Now, it's obviously unusual to call attention to your supporters' low level of education—it's normally a seen-not-heard situation. But Trump, who graduated from an Ivy League school, also seems to be putting himself in this group. We're smart and loyal, he says to his kindred spirits, We're just proud Americans with common sense. Trump also has plenty of reason to love the less-educated: the half of voters without a college degree he took in Nevada (including 57 percent of those with no college at all) was right in line with what he's been doing across the country.
So, the negative interpretation is, first, that Trump loves "poorly educated people" because they would be easily convinced to vote for him, in an assumption that these "poorly educated people" will be "dumber" that people with more instruction (I'm not saying they're, but it's implicit in the argument). So he's admitting that his proposals are not so good, and that individuals who are able to properly understand about politics will be less prone to vote for him. However, according to the interpretation in the Huffington Post article, this is an implicit prejudice against the "poorly educated".
Also, it's not clear who is "we" in "We’re the smartest people, we’re the most loyal people", but an interpretation (the one of the Esquire article) is that this "we" is the "poorly educated". So, according to this interpretation, he is not only falsely including himself in this group but also saying that education makes a people less smart and less loyal; this can be seen as an anti-intellectual view.
So, it seems that the statement is mainly mocked as an admission that less educated people will easily vote for him, and as an anti-intellectual prejudice.