When my battery capacity is 75 is it bad or good

It is bad, and since the battery has reached the critical point of its life, it should be replaced as soon as possible, to avoid unpredictable behavior.

Create a full backup of your device (using these instructions:--> How to back up your iPhone, iPad, and iPod touch), then make an appointment (*) and have your battery replaced at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

if you mean battery health then they advise people to replace the battery when it gets lower than 80%