Is a mri usually done after having knee surgery especially if the person is still having problems?

Not uncommon: It would not be uncommon to image the knee in this situation and MRI is often the best test for this.

Depends: This depends on the nature of the knee surgery. Was it just debridement of torn menicus or more extensive surgery? A follow up MRI may be useful to assess for interval change or for new findings, including retear of meniscus, displacement of a joint cartilage, infection, etc.