In summary
There are several things you can do to improve your Service Design portfolio:
Ask someone who has hired designers in the past to review it.
Watch or read public portfolio reviews.
Look at the portfolios of other service designers to get inspiration.
Ask someone who hires to review your portfolio
You can easily find people who are willing to spend time mentoring you on the website ADPlist. There you can filter the mentors by job titles to find only those who might have hired service designers in the past. For example:
Sites where you can ask for portfolio reviews
Below you'll find websites where you can send your portfolio and get back a critique of your portfolio (in no particular order). These websites focus on UX portfolios but the feedback can still be helpful.
Marie Shaw (99$)
Ian Fenn (99$)
UXplaybook (99$)
Scott La Counte ($70)
The Designer Toolbox (99$)
Public portfolio reviews
I have added for you a few public portfolio reviews. These reviews are of UX portfolios, but there are still lessons to learn.