Well. You get what people usually get, e.g.

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0107259

I agree. Having extracted mIR from plasma you tend unlike plasmid or genomic DNA to obtain low yields and also low 260/280 and 260/230 values. As Long as this yield fig is consistent - and those values are not unexpected for sample source and species - I would not worry and proceed as normal. Plasma extractions of small RNA species unlike DNA or RNA from whole blood are never text book