Pretty much every historian who comments on Howard Zinn start by explaining that all history has an agenda, it's about interpretation of messy and patchy records of human thoughts and actions. Howard Zinn is very up front about his. He literally writes what the agenda is on the first two pages of his introduction.
Historians will point out disagreements they have with his interpretations and work, but will also say it's a very important perspective to have and it has largely been ignored or siloed off in academic specialties of labor history, indigenous history, or ethnic studies.

Most of the people I see bag on Zinn are newspaper pundits and not historians.

That said, A People's History of the US isn't an academic book, so historians don't spend a lot of time with it b/c it's not relevant to their work.