Topsy is a closed-minded character who can act like a jerk at times. He may have tunnel-vision and refuse to see others' POV. I can agree that he isn't exactly the most friendly of the adults.
However, even with that said, I still don't see him being so stubborn that he would stop attempts at finding a solution just to maintain power. I can see Topsy shooting down the suggesting, but only because he might feel there really is no solution. He might think that the water would come back on its own or something. But that doesn't mean he'd stop people from trying to check it out. He might be skeptical, but he wouldn't go out of his way to prevent anyone from looking.
Also, Topsy is just one dinosaur. Let's say for matter of debate that he is power-hungry. Let's say that he did want to keep control and that was the only reason he took charge. There is one particular problem that presents itself if we look at the situation in this matter:
Topsy would not have control for long.
The drought lasts for a while. I don't know how long, but it was apparently long enough that plants were drying up. And in all of this time, Topsy has managed to regulate the water. This isn't so much because Topsy is in control, but more that all the adults agreed to the rationing. Topsy is merely making sure that everyone drinks only their share of each passing day. The worst he may have done is let threehorns drink longer.
But if he was mandating for power, just how long would it have lasted? If he had truly been hoping for absolute control, then he would have been defeated pretty quickly. He is outnumbered and, even if he got other threehorns of the valley to fight alongside him, they still would not be successful. They might have victory over some residents, but the sheer numbers would get to them eventually. Then there's the two old longnecks, who tower over and could do some serious damage with those tails. Littlefoot's grandparents are not to be trifled with.
So in the end, if Topsy's ultimate plan was to gain power, it would be rather short-lived the moment the residents realize what he's doing. Once Topsy shifted from 'preserving the water' to 'controlling everyone like his pawns', he would have his ass handed to him. Topsy might be stubborn, but he isn't stupid. If he truly wanted power, this is not the way he'd go about it.
But like I have said before, Topsy never once struck me as a power-hungry character. Wanting validation is not the same as wanting power. Wanting people to agree with him is not the same as wanting to control them. I can agree with Topsy being a bit of a jerk at times and easily swayed by his anger, but it's very difficult for me to find any evidence supporting him wanting power, or even having the capability of holding that power.