I know this is an old nerco thread but I've once mentioned that in this time and age, the original game just hasn't aged well at all and there's very little incentive to play through it anymore compare to the rest of the series. I'm not saying its bad, but it is also not something I'd ever recommend to a newcomer of the series to start off, since many of the later games just improved on the formula greatly. Anytime anyone thinks of Wario Land it is (almost) never the original game, its always 2, 3 or 4 for many good reasons. I played through it once and did a few runs of it, it has a bit of charm but it just doesn't hold up, Virtual Boy Wario Land which is more of a direct sequel to the original game really fixed a lot of things the original did poorly on.
It only sold well because it was riding on the success of Super Mario Land which had sold around 18 million copies with Six Golden Coins coming in around 12 million copies sold. It did borrow quite a bit from the Mario (Land) series and didn't quite have its own shape quite yet, which is something that didn't really happen until Wario Land 2. Keep in mind the Gameboy was an extremely popular device that lived on for EONS....even outliving the gameboy color....I feel like someone is going to bring up Yoshi's Island but to be fair Yoshi's Island came out near the end of the SNES life cycle and N64 was already being hyped up prior which is what probably hurt the sales on Yoshi's Island.
If there was ever a Wario game that needs a remake/remaster it would be the original game. Potentially it would address many things that would certainly help it and I'm sure it would standout quite a bit.
So, is Wario Land 1 underrated? No but it isn't overrated either....its more of a sleeper hit, but I would consider Virtual Boy Wario Land and maybe Wario Land 2 as underrated.