Mikan does have feelings for Rito but she hasn't fully realized her romantic feelings yet. Yabuki Kentaro, the creator, shows that Mikan will be included in the harem plan in many imagination scenes. Also in the manga, Yabuki Kentro included a poll on whether Mikan and Rito are blood-related or not. I'm pretty sure they are and I hope they are but them being not blood-related has been brought up multiple times that is may just be a possibility. I also ship them and it turns out that Yabuki Kentaro's favorite character is Mikan(Mikan is also my favorite girl as well), Mikan ftw. But to answer your question, yes she does have romantic feelings for Rito!