Many people describe imperfections, deficiencies, weaknesses, etc. as opportunities for improvement. The term is especially prevalent in quality management.
They do this "to express that imperfection is actually a good thing," looked at the right way.
Also as per your question, opportunities for improvement "means 'not perfect' and at the same time conveys encouragement and acceptance."
If you google opportunity(ies) for improvement, you'll get many hits, e.g.:
Opportunities for Improvement - OFIs (Assessment Feedback Comments)
The term "opportunities for improvement (OFIs or OFI)" refers to
written assessment statements (comments) included in Baldrige
assessment reports (e.g., individual-, consensus-, and site
visit-level reports and Applicant Feedback reports) that describe
[Criteria] requirements that are not addressed (i.e., gaps) or that
could be more effectively addressed through process or illustrated
through results. [Baldrige Performance Excellence Program]
The term has much broader and less formal currency.