What feelings does Mercutio reveal for Tybalt in Act 2, Scene 4 of Romeo and Juliet?

Mercutio's feelings about Tybalt are a cross between respect and contempt. He admits that Tybalt is a fantastic swordsman and rightfully fears his power in this area:

O, he is
the courageous captain of compliments. He fights as
you sing prick-song, keeps time, distance, and
proportion; rests me his minim rest, one, two, and
the third in your bosom: the very butcher of a silk
button, a duellist, a duellist; a gentleman of the
very first house, of the first and second cause:
ah, the immortal passado! the punto reverso! the
hai!

This passage shows Mercutio's respect for Tybalt as a physical threat and even creates some suspense in the fight between them later in the play. Mercutio is generally a scornful fellow when it comes to everyone else, up to and including his own friends, so for him to admit anything good about an opponent is more likely to be taken seriously by the audience. However, this does not mean Mercutio completely finds Tybalt worthy.

By referring to Tybalt as the "Prince of Cats," Mercutio is lacing his compliments about Tybalt's fighting prowess with mockery. The Prince of Cats was a figure in medieval folklore, specifically the cycle of Reynard the Fox. These tales featured anthropomorphic animals, including a character called Tybalt, the Prince of Cats. Mercutio is comparing Tybalt to an animal known for its temperamental qualities, thus trivializing his hot-headed nature as childish. He also mocks Tybalt's fashionable use of foreign terms in common speech.

So, it can be safe to say that while Mercutio is aware that Tybalt is a dangerous person, he also finds his pretensions and temper silly enough to mock in public.

In William Shakespeare’s Tragedy of Romeo and Juliet, the conflict that sets everything into motion is the long-standing family feud between the Montagues and Capulets. If it wasn’t for this feud, Romeo and Juliet would have no impediments to their ambitions, and their eventual deaths probably would not occur.

The feud is so important to the story that Shakespeare actually opens the play with it, as men from both houses encounter each other in the streets of Verona and very nearly come to blows.

In act II, scene IV, Shakespeare continues to develop the feud as Mercutio, having just learned of Tybalt’s intention to duel Romeo, speaks of Tybalt in terms at once complimentary (regarding his swordsmanship) and disparaging (calling him “a very good whore”).

But note that before he does that, he first addresses the idea that Romeo has already been defeated by Rosaline, because he is helplessly in love with her:

Alas, poor Romeo! He is already dead, stabbed with a white wench’s black eye, run through the ear with a love song . . .

Mercutio describes Romeo in terms we would normally expect to hear in a situation of violence and murder. It is almost as if he is saying that it doesn’t really matter what Tybalt wants to do to Romeo because he is already a lost cause.

The fact that Mercutio does not yet know that Romeo has forgotten Rosaline and fallen in love with Juliet does not bode well for Mercutio’s future, as this will end up creating the situation that leads to his death.