To start, the mystery dungeon games use the same exp, level and learnset system as the main Pokemon games. As a result to this some Pokemon will level up faster then others because they need less exp to get to lv.100
Pikachu only needs 1,000,000 exp to get to lv.100 whiles Mudkip needs 1,059,860 exp to get to lv.100. As a result to this Pikachu will lv up quicker then Mudkip but if your partner was for example Larvitar then Mudkip would level up faster since Larvitar needs 1,250,000 exp to get to lv.100