Why is it meant to be taken seriously? Why is solo queue meant to be taken seriously? Why is normal draft and blind pick not meant to be taken seriously? What dictates the seriousness which players should approach one queue over another? There's nothing by Riot that says "normals are for fucking around and playing dumb shit, ranked is for going all out 100% to win." What if I don't want to take solo queue seriously? What if I value how I function in a team based environment with people I know and can coordinate a game plan in advance with, more than how I function in an environment with 4 people I've never met and will never see again?
As a matter of fact, what if I don't take RANKED seriously at all? What if I, for whatever reason it doesn't really matter why, decide that I put more stock in normal games than that of ranked games? What if I take normal games seriously but I don't really care about ranked games, and go into them not playing seriously at all?
The whole idea of any queue being a more or less serious queue than another is entirely subjective to the individual players and, quite frankly, stupid and should mean nothing to you. You shouldn't expect anyone to take any queue more seriously than the way in which you approach whatever queue you find to be the least serious.
And furthermore the idea of "serious" itself is entirely subjective. Some people take "playing seriously" to be not playing a champ for the first time, others take it to be having played it at least 20 times in normal games, others think you're playing seriously even as a first time mid lane bard somehow.