To add to this conversation a bit - Trump has been caught on tape admitting to publicly and repeatedly lying about all kinds of things, large and small. So yes, he absolutely has the capability to lie - and he knows he's lying much of the time. Hence, it's fundamentally wrong to assume that concern for the truth plays any real role beyond opportunistic, especially in his public statements.
Separately, pushing the view that Trump is just so stupid that lying doesn't really apply to what he says comes across as an attempt to exonerate Trump and potentially present a defense for various related things. Trump is evil and much of what Trump actually pushes for and works to do is both evil and harms the US (and those who voted for him) in a number of ways. Further, choosing and voting for Trump was effectively an admission that the Republican Party is outright evil and works to harm the country and is fine with that. Believing that he's too stupid to even understand that he's lying is one of the routes to try to avoid dealing with that and needing to reflect on and find and dig out the flaws in one's worldview.