I think Merrick's death did affect Buffy. As did everything that happened her first year as the Slayer. Fifteen was a hard age for Buffy. She suddenly finds out demons are real and it is her responsibility to keep the world safe from them, she tells her parents and they force her into the psych ward for three weeks, Merrick is killed by vampires, she loses all of her friends or rather she learns she never had true friends, she gets kicked out of school and no other school in the district will take her, her parents get divorced, she has to go out every night patrolling for demons despite the fact that she is learning how to fight as she goes along (unlike previous Slayers who have years of training beforehand), she has to let her parents think she is a violent gangbanging arsonist psycho because they somehow think that is better than an ethical demon hunter, and she has to uproot out of the city she was born and raised in and move two hours away to a small town she's never heard of with students and school officials that will have already heard of some of the things she has done and thus treat her badly.
It is no wonder why she was so hesistant in Welcome to the Hellmouth to jump back into the fray and battle evil. Her first day at Sunnydale High had the principal talking about her "absymal" transcripts, students gossiping about what a freak she is, vampires once again on campus (just like at Hemery, leading to things like burning down school gyms), her mom dropping her off while saying "Try not to get kicked out", horrible nightmare prophecy dreams about some of the things that are in store for her in this new town, etc.