If it's an unpaid performance then by all means there should be a collection. One club I used to play at was free admission, but everyone would be expected to put (at least) the cost of a pint in the pot, and the bar takings would top it up to a round figure. Often that translated into more for the performer than they'd have got if everyone had had to pay £2 to get in.

If a performer's being paid, then soliciting tips suggests that something's wrong with the fee, the admission price or the promotion.