It's perfectly fine. Even Apple says so.
12 watt charger compatibility
If you are using an iPhone 5S or older, it will only pull a maximum of 5 watts from the charger, so you won't see any decrease in charging time over a stock 5 watt charger.
If you are using an iPhone 6 or 6+, the phone is capable of drawing more power and thus decreasing your charge time. It can and will get hotter during charging and this is normal.
The device itself dictates how much power it will draw. If you had a charger capable of outputting 50,000 watts- the phone would still only draw 5 watts (in the case of the older phones, etc.)