Hi anjuraas , you should elaborate more your questions, providing more details (e.g. the commands you used, summary output of bowtie2) and the reason why you think 90% mapping rate is not expected.
miRNA mapping rate may vary for a lot of reasons. The paper Evaluation of microRNA alignment techniques has some hints on some of the reasons. In general, I would expect: high mapping rate, but also high multi-mapping rate.
someone told me alignment rate of miRNA should be below 50%. so I got confused.
And why it should be bellow 50%? "Someone told me so" isn't a good answer. Indeed, your mapping rate seems too high, I think more common is something in the range 50-75%. You can map your miRNA against miRbase, this will provide a quality control and annotation of your miRNAs.