I think there almost certainly was some fraud, but not enough to change the outcome. (Whether or not "some but not enough to change the outcome" counts as "widespread", I don't know.)
But I also think it is true that the US electoral system is very poorly organised, creating a lot more scope for fraud–or at least the appearance of fraud, or the appearance of the possibility of fraud–than in some other countries. In a number of other countries, national elections are fully run by an apolitical independent national agency (e.g. the Australian Electoral Commission), not by a hodgepodge of state and county officials many of whom are politicians or political appointees. That system provides a lot more professionalism and a lot less opportunities for suspicion of fraud than the American system does.
Yet, I think some people are so committed to "everything Trump says is 110% wrong" that they want to shut down discussion of the flaws in the US election system. It is possible that both there are real flaws in the election system, including flaws that permit fraud or the appearance of a possibility of fraud, and that simultaneously those flaws while very real and in need of being addressed didn't change the outcome this time around.
Possibly, one outcome of this election might be support from Republicans for some reforms in the way elections are carried out. Given America's toxic political polarisation, there is the risk that Democrats will oppose such reforms simply because it is Republicans proposing them.