What is a good alignment rate in the Expression Count report?

Whether an alignment rate can be considered "good" depends on the source and library prep kit used for the sample, so it can be difficult to assign a hard cutoff. That said, you might want to look more closely at samples with less than 70% alignment. 

Keep in mind that the number of reads that end up aligned, rather than the percentage of aligned reads, is often more important. A low read count aligned to the genome can call into question the validity of any downstream results.

A Sankey diagram in the Expression Count (STAR) pipeline provides sample alignment metrics.