I hear this a lot. I've then gone back and listened to the exact words he's said. I think it's a very hard case for people to say there's anything he has said that's actually racist. Mexico isn't a race, and even if it were, he's simply correctly pointed out that illegal immigrants are not the best people the country has.
Trump hasn't said anything anti-immigration. Only anti-illegal immigration. Much of it is focused on Mexico, and opponents yell about how terrible this is. Yet Mexico is hypocritical. I'm in Guatemala, and friends that have simply had a layover in Mexico require a visa and go through harassment.
On top of that, I've been threatened deportation from the US during a border crossing (I'm Canadian). My wife lived as an illegal immigrant in the US (visa overstay while we tried to adjust status). My daughter was indirectly killed due to us having to leave the US and return to a bad country with junk healthcare.
Despite that, I realise mass migration won't help. Hint: Most of Latam has it not-so-good. Probably a majority would jump at the chance to get US residency. But how is that a solution? Not to mention the current status quo mostly benefits Mexico. Guatemalans face a hard time just getting through Mexico - Mexico has harsh immigration against them and won't hesitate to deport.
Trump's opponents refuse to address this. Instead they just scream racist/bigot/immigrant-hater, while waving Mexican flags. More and more people are realising these are empty insults and non-sequiturs.