TLDR don't use Hitmontop.
Hitmontop isn't a terrible Pokemon in a vacuum, but the circumstances surrounding its acquisition make it a nonviable choice for a playthrough of HeartGold and SoulSilver. Tyrogue is only obtainable as a gift from Black Belt Kiyo in Mt. Mortar, which requires Waterfall (found in the Ice Cave) and a significant amount of backtracking. Tyrogue is then obtained at Level 10, requiring ample grinding to bring it up to speed.
Hitmontop is by far the most annoying of Tyrogue's evolutions to obtain, as you have to ensure its Attack and Defense are equal upon evolving at Level 20. This can take a fair amount of trial and error; you will want to save at Level 19 and soft reset if it doesn't have the exact stats you need.
Once you obtain Hitmontop, it has serviceable Attack... which would be helpful if it learned any good Fighting moves. Your options are the mediocre Triple Kick, Rolling Kick, and Revenge (which require the Move Reminder) or Close Combat, which is learned at Level 51 and therefore may not come in time for the battle against the Elite 4. The rest of its movepool is similarly barren, with "standouts" being Strength (HM) and Gyro Ball (Level 42).
Overall you should probably use a different Fighting type like Heracross or Machamp, which are caught earlier, have better moves, and are less annoying to raise. Hitmontop is just not a Pokemon you should be going all the way to Mt. Mortar to get if you value your time.
