Is this Batman cartoon movie considered canon? I've never seen it referenced in the series at all, but that doesn't mean much. Besides, if Diana of Themyscaria is after you, you really don't have a chance. I hadn't watched this movie in years, but while I was without internets and going through my movies I thought I should. It's quite good, but I sure have some questions and comments about the story and the negligible effect it actually had on the DC Cartoon Universe.
The first this is that it was made pre-Harley Quinn. How odd is it to see the Joker without her now? I have never cared for finding out any version of the Joker's true origin, other than he was the Red Hood first. Giving him a name and a background just seems wrong to me, no matter how good the story is.
And I know the Phantasm had a brief cameo in the Batman Beyond episode of the JLU (still think that's an alternate history, I refuse to believe this Bruce who likes Diana would ever turn into such a bitter old poop) but that's it. If Bruce realized he could be happy with this chick, he could realize it with Wonder Woman or anyone else. See my open letter to Bruce Wayne for more on this.
Like I said, it doesn't stop it from being good movie, probably the second best Batman movie out there, but it doesn't quite gel with the rest of that DCU, as presented in other shows, or even later episodes.