Learn Portuguese (Portugal): A aluna é muito feliz.

In Portuguese, using ser with feelings implies a permanent or natural trait. Because we used é, it means she has a generally happy personality. If we wanted to say she is just feeling happy right now in this moment, we would use está (from estar).

Since a aluna is feminine, why doesn't the word feliz have a feminine ending?

Adjectives that end in a consonant, like -z or -l, usually have only one form for both masculine and feminine words. So, it is feliz for both a aluna (the female student) and o aluno (the male student).

Why doesn't muito change to muita to match the feminine noun aluna?

In this sentence, muito means "very" and is describing the adjective feliz. When used as "very," it is an adverb (a word that modifies an adjective) and never changes its form. It only changes to match gender and number when it means "a lot of" and describes a noun (like muita água for "a lot of water").