Aniline on protonation forms anilinium ion, which is meta-directing. So, a considerable amount of meta product is formed alongwith o-nitroaniline and p-nitroaniline.
Nitrating mixture is mixture of conc. HNO3 and a conc. H2SO4. When aniline is reacted with nitrating mixture ortho, meta and para nitroanilines are obtained. Aniline being basic in nature, reacts with acids to form anilinium ion which is meta directing.