Ok i was looking through the coloured death note version and I think we have to include mello in the can´t-sit-normally-conversation too – not just L and near, because (a) he can´t seem to sit still – he is constantly adjusting his seating position sometimes from panel to panel (while the other mafia guys just sit there with both feet on the floor) and (b) he never has both feet on the floor while sitting. Most of the time they are up on the couch table, or he has at least one leg up on the couch/table he is sitting on. The most usual version is him sitting with his legs crossed – i´m assuming because there is no couch table available at that moment.
Anyway, my conclusion, mello can´t sit normal either, he has just found ways to do it that make him look cool instead of weird