Ah Momiij....
A lot of people hate this character because she's a late love rival who further prolongs the time it takes for Heiji and Kazuha to be official.
For me, I like Momiji and as a Heiji and harem fan, I do like it when a character like him have multiple love interests. After all, while Shinichi does have Ran, there are multiple characters who are interested in him ( Sera- who also debuted late in the story, Haibara- especially in the early chapters/ movies, and Ayumi- the forbidden pairing).