I never bothered with Mallow. Peach's recovery spells make her too useful to pass up, and Mario's offensive strength (especially with the Lazy Shell) is extremely high. Geno is generally good at doing damage with his skills, and his normal attack isn't too shabby either. Bowser is just another powerhouse like Mario. Mallow might have good skills but his regular attack is pathetic, and I dislike using characters that can't go toe-to-toe with enemies (one of the reasons I found many sections of FFIV annoying. 3 casters and 1 fighter is NOT the way to go). More often than not Peach will get all of the FP reserves, and Mario and Geno/Bowser (usually Geno) will just pound away at the enemy.
Also, even with the Lazy Shell Armor, if Peach has the Frying Pan she's still doing close to Mario's damage with her regular attack.