Siding with Hillary is hardly problematic. What was problematic is when she said the following about Donald Trump:

“This instinct to humiliate, when it’s modeled by someone in the public platform, by someone powerful, it filters down into everyone’s life because it kind of gives permission for other people to do the same thing. Disrespect invites disrespect, violence invites violence. When the powerful use their position to bully others we all lose.”

That would be fine, but earlier in her speech, she said (earning laughs):

"So Hollywood is crawling with outsiders and foreigners. And if we kick them all out you’ll have nothing to watch but football and mixed martial arts, which are not the arts."

Apparently Meryl Streep, a powerful public figure on a public platform, can mock and malign a group of people with no apparent sense of irony.