Recently, I've had an idea for a casual type game using similar mechanics to Angry Birds and Cut The Rope which are mobile games. However, the input that I had in mind is suited for a PC. So, I was wondering, how successful are casual games for PC?

According to the synopsis of this research:

https://www.parksassociates.com/report/casual-gaming-market-update

for online casual games, it seems that the market is growing. That is not to say, however, that the offline-single-player casual games are suffering.

Or, if you want to get more details, you can pay $3K to get that report.

I was wondering, how successful are casual games for PC?

More seriously, that's the very beginning of some fundamental business market research. You need to know who your customers would be, what they are likely to buy, what they like and dislike, how much they are willing to pay for a product, and if you can produce and distribute that product for less money than they collectively will pay.

Everybody will have their own answers to those questions. One company (say a major studio) might estimate that leveraging their existing customer base they could get a half million purchases at $20 each and allocate a $8M budget for development. Another company might estimate that they can get 50,000 downloads at $5 each, and make plans for a $100K budget game. Another might estimate that they'll have 500 downloads and $0.30 from ads per player, but since they're making the game on their own they'll invest far more than they will ever recover.

Your numbers will be unique to you.