Are incline push-ups more difficult than regular push-ups?
The incline pushup is actually a much lighter movement as you have to deal with a smaller fraction of your bodyweight, thanks to your body being more upright. Thus it's safe to say, the higher the incline, the lesser the load. Due to the way you are angled, you can get a little more activation of the lower chest fibres, but I feel this advantage is neutralised by the fact that you are dealing with less load. If you can bang out more than 5 clean dips, incline pushups won't really be helping you with muscle-gain.
If you have been training only regular pushups, right from the start- you may find the incline to be slightly more difficult as you're not used to the movement. If you use push-up bars, the incline allows you to go deeper than usual and if you don't generally train with the full range of motion (ROM) you may find the incline to be harder. Other than these cases, the incline pushup is always easier than the regular push-up.
In fact, this is the reason it is used as a training tool. If you are learning a new push-up variant, it is advised to first master it with a high-incline setting, then an incline-setting and finally the regular setting. Using this method, I was able to learn several variants such as the diamond, the wrist, the clap, the tiger-bend and the planche-lean push-ups. Addedly some of calisthenics athletes I follow claim that an incline pushup with forward lean, closely mirrors the tuck-planche pushup. After I have learnt the planche pushup, I will update this answer about how useful the incline pushup was to me XD.
In conclusion- Incline push-ups are easier than regular push-ups, but they offer some lower-chest activation and are an effective training tool