Does mmr even matter anymore?

Does mmr even matters anymore? I'm Silver 2 and sometimes play with friends. I usually score a +18 on wins when soloing, according to op.gg my mmr is hovering at 1420 which is slightly above average for my rank. My friends are lower rank than me and most are Bronze 3-5. When I dou with them I end up still gaining 18 lp rather than 15-16. This leads me to believe that mmr of the game you won doesn't really affect your lp. Hopefully someone can tell me why this is happening. Thanks, Domingo