Kevin Hart -- Mayweather Is Too Good ... Will Somebody Please Challenge the Guy?!

When the hell is someone gonna give Floyd Mayweather a run for his money??

That's what Kevin Hart wants to know ... who tells TMZ Sports Floyd's opponents simply don't have what it takes to test the champion.

Hart -- who was in Vegas to watch Floyd dominate Marcos Maidana (again) this weekend -- told us Floyd's domination reminds him of another legendary fighter ... himself.