Unfortunately, that budget is really limiting. Most beginners don't get that background blur comes from a wide aperture, and kit lenses just aren't as good as expected for this. You need a lens with a low f-number like f/1.8 to isolate your subject. The Canon EF 50mm f/1.8 STM is pretty much the only thing in your price range that can physically create that shallow depth of field you're after.

In my experience, these two options work well:

• Canon EF 50mm f/1.8 STM: provides superior focus.
• Yongnuo 50mm f/1.8 II EF: budget alternative. Both isolate subjects perfectly.

TL;DR: Get the Canon EF 50mm f/1.8 STM. When I first started, I was stuck with flat photos until I bought this classic nifty fifty. I was so satisfied with the creamy backgrounds it gave me for my sisters graduation last summer. It works so well on the 90D and stays under that 150 buck budget... you wont regret it.

To add to the point above: I think you might want to consider a used Canon EF 40mm f/2.8 STM pancake lens. It is built much better than the basic plastic options and the autofocus is very reliable on the 90D. Be careful when buying second-hand tho... make sure to inspect the glass for fungus or scratches because that will definitely ruin your graduation shots. I would suggest staying away from some of the third-party brands if you are worried about reliability. They can be hit or miss with the focus motor and you dont want the lens failing during the ceremony. The 40mm f/2.8 gives a very natural look for portraits and is incredibly sharp even wide open. Just keep in mind that f/2.8 wont be quite as blurry as f/1.8, but it is still a massive upgrade over a kit lens for isolation. You could also look for a used Canon EF-S 60mm f/2.8 Macro USM. It is technically a macro lens but it makes an amazing portrait lens because the glass quality is so high. Just make sure to test it in similar lighting to the graduation venue before the big day so you know exactly how it behaves.

Works great for me