What is Sir Toby's opinion of Olivia in Act 1, Scene 3 of Twelfth Night by Shakespeare?

Sir Toby feels that Olivia should not be mourning for her brother still. 

Lady Olivia has recently met with much tragedy.  Her father and brother both died.  As a result, Olivia is in deep mourning.  She will not even think of looking at a man for seven years.  Sir Toby, her uncle, does not feel that this is healthy. 

SIR TOBY BELCH

What a plague means my niece, to take the death of
her brother thus? I am sure care's an enemy to life. (Act 1, Scene 3) 

Sir Toby certainly knows how to enjoy life.  He spends his time carousing, drinking and singing at all hours.  He hangs out with his friend Sir Andrew, who is sweet on Olivia, and Olivia’s servant Maria, whom he is sweet on, and the fool.

When Maria tells Toby that Olivia does not approve of his drinking and carrying on, he does not really care.

MARIA

By my troth, Sir Toby, you must come in earlier o'
nights: your cousin, my lady, takes great
exceptions to your ill hours.

SIR TOBY BELCH

Why, let her except, before excepted. (Act 1, Scene 3) 

Toby feels that his niece will be accepting of his antics.  He is repeatedly warned, by Maria and Malvolio, to behave more appropriately for her benefit.  He does not change.  He knows that she will never throw him out, because they are kin and she is fond of him.

Toby and Olivia represent opposites here.  Olivia is reserved, and wants to withdraw from life after having experienced so many deaths close to her.  Toby feels that life is for being enjoyed.  If you can’t have fun, what is the point of it?

It takes Cesario (Violoa) to draw Olivia out of her shell.  She falls head over heels in love with this young man (who is not a man), despite herself.  She surprises even herself!