Mewberty Star is the most superior waifu one can ever hope to find.
She retains all the cuteness and adorableness of regular Star, but also has a few improvements.
Six arms mean three times more intimate hugs, and three times more things she can simultaneously do to you in bed.
She has bigger wings that can actually fly, so that she can show you around the world while holding you tight with her three pairs of arms.
I'm not sure if she would permanently have a more horny attitude if she got permastuck in Mewberty form, but either case can be seen as an advantage.
And either way, she would still be a magical princess hellbent on interdimensional adventures and the most fun person and closest friend you can ever have.