True, but Evidence Rule 608 gives you an opening
(a) Reputation or Opinion Evidence. A witness’s credibility may be attacked or supported by testimony about the witness’s reputation for having a character for truthfulness or untruthfulness, or by testimony in the form of an opinion about that character. But evidence of truthful character is admissible only after the witness’s character for truthfulness has been attacked.
That only applies to a witness’s credibility, and not a defendant’s guilt.
You could use 608(b) if Trump agreed to testify, on cross-examination the prosecutor could bring up those past mistruths and it would be allowed. But I think it’s kind of a foregone conclusion that if a lawyer allows Trump to testify in his own defense in a trial, it’s going to be really, really bad.
That’s not really the argument, though. It’s not that he lies regularly, it’s that he is a good actor and can create a show of complete sincerity, on cue. If the defense is that he’s sincere about believing that he was cheated, then showing that he can fake sincerity would be very pertinent. Likewise, if you could show that he had attended acting classes, performed in movies, etc.
And that is what your cite says. If the defense’s argument is honesty, sincerity, respectability, etc. then you can offer evidence to counter it.
Yeah, if a defense lawyer’s angle is that Trump must be innocent because he’s a respected person with impeccable honesty, that’s a huge blunder. That indeed is tossing a softball for the prosecution.
A more plausible defense is just that you can’t prove he did it, and/or had criminal intent. That’s what I’d expect from a legal team, if Trump were able to hire someone other than the clowns he’s been using lately.
Sorry, I largely rewrote my post on a rethink…
I do that sometimes too, it’s all good. 
I think my reply still works for your revised post.