it can't be Mheetu. lionesses don't have another litter of cubs until their previous litter enters adolescence (except in cases where their cubs are killed, like when another male takes over). and Mheetu was meant to be in the movie during Simba and Nala's adolescence.
another thing, it's clearly a female cub. Disney put whiskers on male felines. most believe its Nala with a coloring error on her toes and eyelids (they're too dark for Nala). but I just think its a random female cub.