It probably has to do with Lilipup being a generic early-route-normal-type. Quite frankly, I don't find it particularly cute, if only because it looks like it had a fight with a Swiffer duster and lost. Herdier is somehow even more generic looking than Lilipup, and seems to have decided to go for round two with the Swiffer duster. Stoutland at least gets points for having a glorious mustache, and looking decently groomed.
I have to disagree here. Quite frankly, while gen 5 DID have some unique ideas, I found many of the pokemon in Gen 5 to be rather disappointing. With the exception of Lilipup, most of the early game 'mons are pretty terrible, even from an ingame standpoint. Patrat is slow to level up, has a bad evolution, and is really creepy. Pidove is lame, weak early game, and has one of the worst stat-to-movepool relationships in the series. Purrloin needs no introduction, the monkeys are gimmicky and not great, and Roggenrola was good(and hideous), until it didn't get to be a gigalith.
Basculin manages to be BOTH a generic fish thing, AND useless compared to every other water type in Gen 5.
Let's not forget zubat's worse, more annoying cousin, Woobat.
The Klink line is cool in my opinion, too bad it has no movepool to speak of.
Tynamo evolves into something powerful and quite good, but you have train the thing(and its terrible moveset) first.
I could go on, but I do have things I liked about Gen 5; but a good roster? Not really