I think it's lovely and melodic, but it looks like the epitome of an exaggeratedly frilly princess-y name that only a minority of women would enjoy bearing throughout their lives. An upside is that there is a massive selection of nicknames to be derived from it (Maria, Mary, Mari, Mara, Bella, Belle, Ella, Elle, Ellie, Mabel, Mabelle, Mabella, etc.).
I like the name, but it's not something I would use on a human. I might name a doll or a OC Mariabella but not a real person. I love -bella names so I am inclined to like it, but I don't love the double 'uh' sound. I would prefer Maribella or Mariabelle since they don't have it. It's unique but easy to spell which I LOVE. It doesn't suit middle names either, so I like it okay, but it's not like, my favorite thing in the world if that makes sense.