Not really.
Mid-terms are historically a shit-show for the in-power party. Typically the president's party loses seats and regains them when the opposite party takes over presidency. Despite this, most presidents are able to win a 2nd term.
It is acutally not that common. in 94 republican won. but in 98 off year election democrats gained seats (people's backlash against clinton impeachment)
in 2002 bush and company gained seats as they politicized the iraq war. in 2006 democrats won big iraq was going really bad.
2010, 2014 republicans did well.
in 2018 i suspect republican well gain 2 senate seats.
democrats will gain 20-25 congress seats.
That conventional wisdom of party out of power wins was for when Democrats and republican had roughly equal number of white people voting for them and democrats had a ton of old people voting for them.
but that is not longer the case. white folks are more likely to vote. religious people are more likely to vote. and old people are more likely to vote.
if you look at the Venn diagram Republican dominate those three categories.
in presidential year elections democrats are structurally more likely to win the populate vote. but still loose the congress and certainly lose the a ton of state legislatures.
in the coming years America's political system will continue to its dysfunctional. hopefully populations moving around the country will balance out the states a bit more. living in the south it is just weird.
you drive 30 minutes out of big cities and people are living in a compltely different lifestyle. the social divide in this country is ripping it apart.