The has been a real problem with staffing of ATC centres, which has coincided with an alarming rise in near-misses at US airports. There are many articles online speaking about it and saying that an accident was going to happen the way things were going.
The thing Trump is on about is genuine, as you've seen very qualified ATC operators turned away because of the colour of their skin (they were white). This goes back some way, and some of the policies have since been overturned. There is actually a lawsuit representing around 3000 ATC controllers who were rejected, and ironically that is exactly the same number CNN (the biggest Trump critics of all) say the US is short of.
There were some examples of the FAA temporarily putting the brakes on hiring so it could replace 'race-blind hiring rules' with a 'Biographical Assessment' stratagem designed to hire more minorities. The criteria to hire ATC staff changed dramatically, as the questionnaire sought irrelevant information. By contrast, holding a pilot’s license — a major advantage for a controller — was worth next to no merit. And having valuable experience as an air traffic controller in the military was actually worth no points at all on the assessment.
I'm not saying if this is related to what happened or not, it's far too early to say - which is why what he said was stupid and insensitive. If you are going to have a go at Trump (and I certainly agree some of his ideas here are wrong) then there should be a wider level of challenge, as there is reason to be objective about these things above. There shouldn't be any compromise when it comes to things like air safety.