query related to downconversion mixer

As no scale is shown I only can tell the signal shows some sinusoids. Maybe your RF or LO signal carries some modulation?
REmember - all mixers are nonlinear elements that generate MANY harmonics and frequency combinations. We then use filters to select what we need. Only mixer +filter combination gives a useful device.

Use a Spectrum analyzer to see the output harmonics and spurious responses of the mixer,

As mentioned by jiripolivka the pure sine wave signal can only be seen in the oscilloscope

if all other components are removed or reduce to a power level of 50dBc you will get a sine wave without any problem.

agree with above posts
also check your rf-if and lo-if isoaltions

place a LPF to IF output at your SCh design to debug