Yes, that's exactly it. She's NOT that good at it. (In her position - or even with a much lower level of fame and wealth - then I would definitely employ a stylist. I love clothes, but I'm not as good at them as a professional stylist would be, and it would save me time as well.)
It's all of a piece with her general attitude to a lot of things. She can put flower sprinkles everywhere, so she must be talented enough to star in her own cookery and lifestyle programme. She can post on social media, so she must be able to do the marketing for a jam company and make it go global. She can sit with people and talk to them, so she must be able to create an awesome series of podcasts. She can do a bit of calligraphy, so she must be a design genius. Etc, etc, etc.
It's not that she's laughably terrible at all these things (in my opinion); I think she is actually better than average at all of them. But she's just not that good. Not nearly good enough to make it as a professional. As Harry's wife, she's been surrounded by people who praise her minor talents and who do their best to help her parlay them - along with her fame - into something successful (because it would benefit them too), but all their help hasn't been enough.
The problem lies in the gap between her actual abilities and her own opinion of those abilities. And (presumably as a result) her reluctance to accept advice or to let go and delegate even small decisions. Because nobody can do things as well as Meghan. (It also means that she spreads her time and attention so thinly across a range of activities that she never actually puts the deep work into becoming truly good at any of them.)
I would think this is the real reason behind the lack of stylist. A stylist isn't even that expensive.