I don't know the series well, but from here (mostly literally translating)
Loli-senpai is a nickname for Kana Arima.
It suits her because of her image as a genius child actress and her height being small (150cm=4.9ft).
From the Japanese language perspective, I'd guess it is made from the (kind of) opposite ばばあ先輩 (babaa senpai), which is an internet slang meaning generally older women co-workers who is senior to you. Babaa is a disrespectful way of calling (grand)mother, but used to call women in general (you'll be slapped in the face if you use it in reality).
'Loli' comes obviously from lolita, but here it does not necessarily contain disrespect, it should simply refer to the character's physical features/appearances being like a little girl (at least to the character who gave the name to her). Also, her attire in the link above is similar to typical primary school uniform (not exactly the same though).