The average flip is 31, if you don't include the two times it didn't flip. Obama lost a total of 76 house seats over his two midterms, the highest loss in over 50 years. Clinton lost 54 in his first term.
Running up votes in California and bragging about it as national turnout is what got you into trouble in 2016. It also took an unprecedented number of retiring Republicans for Democrats to flip several traditional Republican seats. Much easier to defeat non-incumbents.
All you can really take away is that turnout was very high for a midterm, and a relatively average flip was achieved. Democrats are stirring up enthusiasm aplenty, but it's not strictly for their own base. They're foolishly rallying Trump's base as well with their constant hysteria.