Sinbad -- Kobe Bryant DISSED Los Angeles ... With Boston College Visit

Not everyone's thrilled with Kobe Bryant's impromptu college visit in Boston last night ... in fact, Sinbad's downright upset -- telling TMZ Sports it's a huge INSULT to the schools in L.A.

Sinbad -- who lives in Los Angeles for decades -- pointed out that Kobe could've sat in on classes at UCLA or USC during his 18-year stint with the Lakers ... and didn't have to fly all the way across the country to learn about marketing.