Honestly, the 50mm is fine for a start, but I had issues with my copy when I first got into portraiture on my 90D. The bokeh on the Canon EF 50mm f/1.8 STM was a bit nervous for my taste, and unfortunately, the focus hunting in low light really ruined a few shots during a sunset session I did last year. It felt cheaper than I expected. On an APS-C sensor like yours, that 50mm acts like an 80mm which is a classic length, but the quality just isnt quite there for professional-looking graduation shots. If you can stretch the budget or find a used deal, check these out:

• Canon EF 85mm f/1.8 USM for much smoother background compression.
• Canon EF 50mm f/1.4 USM if you want slightly better light gathering than the f/1.8. I really wish the cheap 50mm performed better since its well under $300, but the chromatic aberration is just annoying... definitely watch out for those purple fringes in high contrast areas.

Building on the earlier suggestion, I totally agree the 50mm can hunt sometimes... kinda annoying tbh! If you want a technical beast for that 90D sensor, find a used Canon EF 85mm f/1.8 USM. Since the 90D is a crop sensor, that 85mm acts like a 136mm lens which gives incredible background compression! The f/1.8 aperture and the fast USM motor is just amazing for portraits.

Unfortunately, I've had issues with the cheaper 50mm primes on that 90D. The high-resolution sensor basically highlights every lens flaw, and the Canon EF 50mm f/1.8 STM just wasn't as sharp as I'd expected. Chromatic aberration is also quite severe in backlit graduation photos. A used Tamron SP 45mm f/1.8 Di VC USD is a much better investment... the optics are superior for that body and wont disappoint.