Floyd Mayweather -- Sucked On 'Dancing' ... Ex-Partner Says

It's been almost 10 years since Floyd Mayweather LOST to Mark Cuban on "Dancing with the Stars" -- and his old partner says they deserved it ... 'cause they sucked!!

We spoke with Karina Smirnoff, who partnered up with Floyd in 2007 -- the year the duo became the 4th couple eliminated ... before Cuban, Jane Seymour and Marie Osmond.

So, what went wrong??

"I think it's a combination of footwork, arm work, bodywork, rhythm."

Ouch.

BTW, we pulled up some of the old video ... they're not THAT bad.

For the record, Cuban got eliminated the next week.