In the games, the regular Diamond Princess Diancie was far less offensive than its Mega form. Its base stat total of 600 barely made itself felt: its extremely low HP undermined both defenses, and most of the time, it could only make use of one of its two base 100 attacking stats. Its low Speed left it with little offensive threat, too. Against a princess that mainly disrupted opponents and pivoted but lacked reliable recovery, a pivot with good recovery could handle it head-on, without necessarily even resisting its attacks. Most defensive Water-types, for instance, were beyond the princess's ability to break through. The same went for defensive Ground- and Grass-types, as long as they could actually hurt it.
Of course, Steel-types were the safest option overall. They were immune to Toxic, could take both its STAB attacks, and hit it with quadruple-effective STAB moves. Regular Diancie currently had a pretty harsh environment to survive in. As mentioned earlier, unless you brought a Flying-type attacker like Mega Pidgeot, Diancie often had little presence in a match.
Looking at Diancie from every angle, without Mega Evolution, it simply was not a Pokémon as dominant as Garchomp. In some respects, it could not even match Dragonite. After Mega Evolution, however, Diancie gained its own tactical niche in the games.
First, Mega Diancie's base stat total reached 700, far surpassing that of some top-tier Legendary Pokémon. Stats like those were an advantage in their own right and offered the potential to raise a powerful Pokémon.
Still, its base 50 HP remained a flaw, and Diancie had too many weaknesses, many of them to types used by extremely popular Pokémon. In UB, it was fragile. Its two base 160 attacking stats could not benefit from an item boost, and it lacked explosive setup moves,