At Lv. 100, Pikachu is, unfortunately, lots worse than other Lv. 100 Pokemon. Pikachu has extremely mediocre stats and its base stat total is 320. Even lower level Pokémon will be significantly stronger. If you want to a solid electric type in battling, either evolve Pikachu or find a stronger electric type, such as Zapdos, Magnezone, Jolteon, etc. If you really like Pikachu and would like to level one up to Lv. 100, then that's perfectly fine. Any Lv. 100 is valuable in my opinion. I mean, it can't be that bad, especially since it can hold Light Ball and Pikinium-Z. But don't expect to do very good in online battle.
Basically, Pikachu isn't very good, even at Lv. 100. But it is still Lv. 100, so you may want to consider taking the time in evolving it, especially since it is such a "beloved" Pokémon.