Ludacris -- Baby Mama Says Quickie Marriage is a Cheap Trick to Win Custody

Ludacris' hastily arranged marriage is a cheap trick in his attempt to gain full custody of his daughter ... so says a source close to his baby mama.

We're told Luda's baby mama, Tamika Fuller, thinks Luda wants to show the judge in their custody fight that he now has a stable, loving household ... more conducive to child-rearing than the home of single mom Tamika. She fears a Georgia judge could actually buy what Luda's selling.

As we reported ... Luda abruptly proposed to GF Eudoxie during the XMAS holidays and married her the very same day. Tamika thinks it was all just a ploy.

Luda is gunning to win full custody of their 1-year-old daughter Cai. He claims he's a better parent and has the means to provide her with a better life.

Tamika thinks it's a crass ploy to avoid paying $7,000 a month in child support.

The courtroom fireworks begin later this month.