FYI: A lot of attorneys aren't listed in the Martindale system for one reason or another.
I am sure tons of small law firms are not listed on Martindale, but the ones (small law firms) that do, are not necessarily only the top small law firms that only hires top law schools graduates or top law students. In other words, they should be reasonably representative of an average or maybe a slight average small law firm.
By the way, do you know how many percent of law firms are not listed on Martindale.com?
Do I know "how many percent"? Did you mean to say "what percentage"?
Anyway, I would estimate the percentage fairly high. Maybe a third aren't listed, based on my personal research experience in DC and the Southeast. Also, amongst the firms that are listed, a number of attorneys within those firms aren't accurately listed, e.g. no mention of honors they received during school, or not listed at all.
If you're at a lower tier 2 or 3 law school, your best shot is to check with your career development office for a list of alumni in those markets. In most cases, I would trust your school's list before Martindale's. There's a good bit of outdated information on Martindale's as well. Good luck breaking into either market.