In order to be a true prophet, a person requires two qualifications. The first is that they must speak in the name of the Lord God of Israel (which obviously involves being consistent with previous revelations about Him). The second is that they must be 100% accurate in his prophetic predictions. Joseph Smith, though he was evidently aware of both tests, failed them miserably. His own statement concerning false prophets condemns him as one.
As an example, in Doctrine and Covenants 84:1-5, we read "A REVELATION of Jesus Christ unto his servant Joseph Smith, Jun., and six elders, as they united their hearts and lifted their voices on high. Yea, the word of the Lord concerning his church, established in the last days for the restoration of his people, as he has spoken by the mouth of his prophets, and for the gathering of his saints to stand upon Mount Zion, which shall be the city of New Jerusalem. Which city shall be built, beginning at the temple lot, which is appointed by the finger of the Lord, in the western boundaries of the State of Missouri, and dedicated by the hand of Joseph Smith, Jun., and others with whom the Lord was well pleased. Verily this is the word of the Lord, that the city New Jerusalem shall be built by the gathering of the saints, beginning at this place, even the place of the temple, which temple shall be reared in this generation. For verily this generation shall not all pass away until an house shall be built unto the Lord, and a cloud shall rest upon it, which cloud shall be even the glory of the Lord, which shall fill the house."
At the tragically young age of 38, Joseph Smith was killed in prison. His death erased all possibility of him ever dedicating anything with his own hand, meaning the prophecy found in D&C 84 failed. Joseph Smith, by both Biblical reasoning and by his own words, was a false prophet. Anyone who loves God must therefore reject Joseph Smith, lest they share in his condemnation.