Milan veteran Pippo Inzaghi cancelled out Hugo Campagnaro's early opener for the visitors.
"After taking the lead, we could've scored more, but didn't manage it," said Mazzarri. "The second half was more balanced, we played at great pace and created quite a few scoring opportunities.
"We forced Christian Abbiati into some crucial saves and if we had scored another goal, we would've deservedly won the match."
Napoli are without a victory in eight rounds and have slipped further down the standings.
"These players give their all, even if at the moment the results are obviously limited in terms of goals scored," continued Mazzarri.
"In football if you create many opportunities and don't make the most of them, you end up paying the price.
"It is important for this season to grow, mature and develop our own style of play while aiming as high as possible. Then we hope to do well on the summer transfer market."