I wouldn't say he's overrated exactly, but I can understand why people would. He's a protagonist of many firsts for the series. Tommy was the first playable character to receive a voice, and a personality at that. The magic of Claude was you could project your own personality onto him through your playing style. Whilst Tommy definitely had his own voice, opinion and morals, you as the player were able to influence this slightly and his demeanour was generic enough that it fit well within the story and setting. You could play it safe, or you could go in all guns blazing. Whatever your tactics, Tommy never really clashed with it.
He stood out too. Fresh from the drab and dreary city of Liberty, we saw sunshine, palm trees, lapping waves and one of the loudest Hawaiian shirts seen this side of Vice. He stood dominant in early screenshots, wielding a katana overhead that played with the sunset behind him. It's easy to see why people put so much into this character, and why he's so well revered today. Things took a step backwards with CJ, presenting him in nothing but blue jeans and a white vest (at first) that mirrored his surroundings well, but did little to show off what made him special. Tommy is the quintessential protagonist for many, and it just so happens you play as him in one of the most beloved games of the series.
To me? I'm not what you'd call a die-hard Tommy fan but I can see why people would consider him so good as a protagonist.