We say "more beautiful" to refer to someone's beauty as being greater than another person's or other people's beauty. Here we use more because beautiful has no comparative form. When we deal with the words that have a comparative form, we use the comparative, e.g., "he looks happier today"; here we are using happier because happy has a comparative form.
But even if an adjective has a comparative form, can we use more to compare with others? Do sentences such as "John is more tall than Jack", instead of "John is taller than Jack" sound odd?