Does more acidity = better electrolyte?
Essentially, 'how good of an electrolyte' something is depends on how many particles the substance dissociates into. Let us discuss acidity. Now, from experimental evidence, it may seem that 'stronger' acids and mixtures, such as HCl, $\ce{H2SO4}$, Muriatic, are all good electrolytes. These are not because of their relative acidities. You will notice that in my last sentence, I put 'stronger' in quotes. This is because when talking about acids, 'stronger' has a different meaning than the one you are used to. A strong acid is one that dissociates virtually 100%. In other words, when you put a strong acid in water, all of it turns into ions, and you have none of the original acid left over. Similarly, a weak acid is one that does not completely dissociate into ions. By the definition of an electrolyte, stronger acids are better electrolytes (more ions in solution). This leads to our next point, ions in solution. As mentioned by Zhe above, the 'Van't Hoff Factor' is important. Essentially, it is just a measure of how many particles a substance dissociates into, for example NaCl has a VHF of 2, whereas $\ce{CaCl2}$ has a VHF of 3. The higher a substances VHF, the better of an electrolyte it is, the reasoning being that with an increase in number, there is an increase in dissolved particles, and therefore an increase in the state of being an electrolyte. Now, going back to acids for a moment, you may be wondering which acids are strong. There are 6, and they are HCl, HBr, HI, $\ce{HClO4},\ \ce{H2SO4},\ \ce{HNO3}$. Weak acids are everything else. These strong acids will have the best electrolytic results if you use them as such, much better than lemon juice (citric acid) or vinegar (acetic acid). In your example, you used muriatic acid (HCl) as one of your ion-producers, which explains the rapid voltage change, occurring from the dissociation of a strong acid. 

$$\text{In essence, the answer to your question is:}$$ $${\text{How many particles a substance dissociates into determines how good of an electrolyte it is.}}$$