grades will kill you no matter what undergraduate school you attended. You are more likely to see high board scores out of Ivy leaguers than others and more likely to see high boards out of BYU than a less competitive school like UVU. There will always be exceptions. Kids can't get into BYU without high ACT and SAT scores, which usually translate into high LSATs, MCAT, GRE, etc.
If you research it, SATs, LSATs, etc are rough proxies for IQ. Most grad schools want higher IQs.