Evie (ChickasaurusGL) 🌺 | Receiving the Cerulean City Bulbasaur happiness gift, even though we never received Pikachu (Yellow) @ChickasaurusGL | Uploaded June 2022 | Updated October 2024, 1 minute ago.
When you start a new game, the happiness (used for the starter/walking Pikachu) value is 90. This is still the case even after performing the 255 Pokémon glitch. https://glitchcity.wiki/255_Pok%C3%A9mon_glitch_(Generation_I)

Interestingly, I remember reading the NPC at Cerulean City will give Bulbasaur if the happiness is 147 or higher if Pikachu is not in the game (and as confirmed here; even if Pikachu was never in the game to begin with; the latter is impossible without glitches).

The 255 Pokémon glitch is an exception that lets you warp to Cerulean City without Pikachu via a Pokémon swap enabling the expanded inventory, then adjusting the expanded inventory (e.g. the item 36 quantity 'exit'). Furthermore, because the top Pokémon have lots of 255 bytes, and the expanded party does map later on to D46F (happiness) https://glitchcity.wiki/Expanded_party you can manipulate the happiness with a Pokémon 18 swap; giving you 255 happiness (for the walking Pikachu/shared with any walking Pikachu if more than one, even though none has ever existed in the game).