Chyna's mom, Tokyo, tells TMZ ... her daughter's on a downward spiral, following recent incidents involving cops at her home and, of course, her short-lived relationship with rapper Kid Buu -- who allegedly got violent with BC. Mom's also concerned about his rap sheet.

Tokyo didn't mince words -- "It doesn't take a rocket scientist to see something is terribly wrong."

As TMZ first reported ... an anonymous person called cops last month, saying Chyna was intoxicated and neglecting Dream. Cops later determined everything was fine ... besides BC having a falling out with her makeup artist.

There was also Chyna's fight with 'Love & Hip Hop' star Alexis Skyy over Rob -- and the cherry on top for Tokyo ... BC's fling with Kid Buu.

You'll recall, she jetted off to Hawaii with her new BF. Our Rob sources said he had no idea, and couldn't reach his baby mama when it came time to hand off Dream. Chyna disputes that.

Of course, Chyna's vacay went to hell when she and Buu got into a fight. Cops were called, but neither was arrested. As we told you, Chyna's now ex-BF has faced allegations of physically abusing a woman before.

All the drama makes Tokyo think BC's judgment is clouded, and it's in Dream's best interest to be with dear ol' Dad.

However, like any mama bear ... Tokyo's still defending Chyna against Kid Buu and has a stern warning for him. Watch.