Is it a felony or can you get addition time if you were sentenced to 90 days in jail but you don't go?

Yes, if you fail to obey a court ordered jail sentence you can get more time, whether that's a felony or misdemeanor depends on your jurisdiction. Making your sentencing judge angry is not a smart thing to do

My answers to questions do not constitute legal advice nor do any answers create an attorney-client relationship.

Yes, if you were sentenced, you'll need to go serve it

If a person fails to turn themselves in, when ordered to do so, that could create a problem. If serving the 90 days is a condition of probation, felony or misdemeanor, failing to comply with that condition (serving 90 days) could result in a probation violation that could result in additional time to serve.