Not exactly. Mallow has very good hit-all attacks if he gets leveled up enough. If you get a bunch of enemies and Mallow has either "Snowy" or "Dark Star" then he will be pretty strong. Combined with his enemy HP searcher and HP rain, he is a very good overall character.
For a good party, you need a good combo of attack AND defense to be worth anything. Smithy is pretty easy for a LV 30 party with Mario, Bowser, and Toadstool. But the one time I beat Smithy with a LV 25 TOTAL offensive party(Mario, Geno, and Bowser), I had a bit of trouble because there was NO healing without use of items(I stocked up on Megalixirs). Sure, it freed up FP for offensive use, but getting all those Megalixirs was a PAIN, when Toadstool's healing abilites could be fully replenished with a single Royal Syrup.