As already pointed out by the other answers, an additional kilogram is rather negligible when concerning purely the additional potential energy you need. But there are other factors where it may have a more or less larger effect.

Firstly that your body does not necessarily respond linearly to higher load. As long as you are in a region where you can do the uphill without noticeable fatigue you will just be a bit slower with the same power output. But if the uphill will bring you to your limits even without the additional weight, every additional gram will bring you to your limit a bit earlier and make it more hard to go on.

Also ass is playing an important role every time you have do accelerate it in some way. Some things I think about are:

• Smoothness of the uphill: The approximation which concerns only the potential energy works the better, the smoother the uphill is. If you go up on a constantly inclined, paved slope where you can assume to ride with nearly constant speed, there shouldn't be much of a difference. However, if you go mountain biking on a way where you have lot of "action" on the bike (climb up steps, try to keep traction on loose ground, switch between steeper and flatter sections), it can be different.

• "Place" of the additional weight: I can make a difference where you add the weight. The largest effect will be if you have additional weight on rotating parts (wheels, drive train). But also a heavier bike itself will make it feel less responsive which will make your muscles go tired faster if you have to do a lot of bike handling stuff during the uphill (therefore it is again related to the smoothness of the uphill). If you had the additional weight in your backpack instead, it would have a much smaller effect.

Conclusion: Just from the additional potential energy point of view it may not make much of a difference but if you concern bike handling or fatigue close to your limits, it will have a larger effect.

As already pointed out by the other answers, an additional kilogram is rather negligible when concerning purely the additional potential energy you need. But there are other factors where it may have a more or less larger effect.

Firstly your body does not necessarily respond linearly to higher load. As long as you are in a region where you can do the uphill without noticeable fatigue you will just be a bit slower with the same power output. But if the uphill will bring you to your limits even without the additional weight, every additional gram will bring you to your limit a bit earlier and make it more hard to go on.

Also mass is playing an important role every time you have to accelerate it in some way. Some things I think about are:

• Smoothness of the uphill: The approximation which concerns only the potential energy works the better, the smoother the uphill is. If you go up on a constantly inclined, paved slope where you can assume to ride with nearly constant speed, there shouldn't be much of a difference. However, if you go mountain biking on a way where you have lot of "action" on the bike (climb up steps, try to keep traction on loose ground, switch between steeper and flatter sections), it can be different.

• "Place" of the additional weight: It can make a difference where you add the weight. The largest effect will be if you have additional weight on rotating parts (wheels, drive train). But also a heavier bike itself will make it feel less responsive which will make your muscles go tired faster if you have to do a lot of bike handling stuff during the uphill (therefore it is again related to the smoothness of the uphill). If you had the additional weight in your backpack instead, it would have a much smaller effect.

Conclusion: Just from the additional potential energy point of view it may not make much of a difference but if you concern bike handling or fatigue close to your limits, it will have a larger effect.