That's not a good argument, since they did call it Dragonball. They did decide to try to make a film adaptation of the series. They did market it as such. But they didn't do a good job. At worst, it's a shitty attempt at cashing in on the popularity. At best, it's a very loose adaptation. Neither are what people wanted to see, so it's a bad film. Dismissing complaints because "It would've been better if it wasn't called Dragonball" is a null argument. Super Mario Bros. could've been a good movie if it wasn't called Super Mario bros. Street Fighter could've been a good movie if it wasn't called Street Fighter. Alone in the Dark could've been a good movie if it wasn't called Alone in the Dark(unlikely, but you get the point.) But they weren't, and aren't.
When you market something towards fans of a series television or book series, it needs to be faithful. You're already taking a hit on ticket sales from people who already aren't interested in the source material. Just because the names "Goku," or "Mario," or "Guile" are being used doesn't boost a script to gold ink on gold paper status.