My perception of it is that everyone got used to him running mid pack for years. Similar to guys like Biffles, Newman, Burton, Kahne etc.. difference is those drivers had good starts to their career and Truex didn't.. So he never built much of a fan base.
Then all of a sudden FRR decided they would like to win stuff sometimes, switched to Gibbs Toyota equipment and began to do just that. Truex's head got big (deservedly so) and we started to see a lot more of him in interviews and being talked about by the guys in the booth etc...
IMO it's mostly that we never saw his passion before.. then he led most of the laps for like 20 races and didn't win anything.. so we got to see him act all 2017 Kyle Buschesque for a bit.. that doesn't help fan perception of him.. he's become a solid front of the field driver but I just don't think he has the fanbase most other drivers in that spot do.
Because of the slow start to his career most likely but also IMO he has shown in recent years his personality is just not one to be favored. He kind of reminds me of what I think it would be like to see Paul Menard win races and.. you know.. smile every once in a while. Cept in a Toyoder.
Personally.. I don't like him. Not for anything having to do with driving.. he's a great driver... I just don't like his personality and attitude and the way he presents himself.