$\cos 90^o = 0$. Since the angle increases from $90 - 180$, the value of $\cos \theta $ becomes more and more $-ve$ and hence resultant decreases. Thus, dipole moment is maximum when $\theta = 90^o $.
$\mathrm{NH}_3, \mathrm{SO}_2, \mathrm{SiO}_2, \mathrm{BeCl}_2, \mathrm{CO}_2, \mathrm{H}_2 \mathrm{O}, \mathrm{CH}_4, \mathrm{BF}_3$
$(I)\, CO^{2-}_3$ $(II)\, XeF_4$ $(III)\, I^-_3$ $(IV)\, NCl_3$ $(V)$ $BeCl_2$