Are there any downsides to using a mountain bike as a commuter bike? I am going to buy a bike soon, and I want to make sure I’m making the right choice.
I will be using it for commuting and light exercise (nothing over an hour of use a day). I’ve been recommended buying a MTB as it can be used for rough terrain and roads. However, I recently read that it won’t give the best performance.
Summary: I want to use a mountain bike for commuting.
Has anyone with a mountain bike experienced difficulty cycling on normal car roads?
And are there any real downsides to this?
Thank you
are there any real downsides to this?

The one thing about a typical MTB that really doesn't work well on the road are soft-compound, deep-thread tyres: they have lots of rolling resistance and wear out quickly. But this is easily fixed because even the burliest enduro bike can be fitted with more road-suitable tyres without problems.
Smaller disadvantages include the higher mass and suspension bobbing. IMO neither of that is really an issue for commuting, though it definitely does mean you get not the best performance. If it's a long commute then maybe you're better off with a more performant bike but for a short commute (&lt; 5 km) honestly just about any bike will do; I'd recommend making longievety / low maintenance more of a deciding factor than performance. Also, a fancy bike is no good if it's stolen.
In summary: an inexpensive hartail MTB makes for a perfectly adequate commuter bike, as well as for “light exercise”. Only if you want to get a bit more adventurous about the MTB part, then I'd recommend rather getting two separate bikes, with a cheap, used, road-only one for the commute job.