No it isn’t, unless the mod in question is under a license that very specifically prohibits redistribution it’s perfectly legal, even if that is the case it’s practically impossible to enforce, there are dozens of website that have been redistributing mods for minecraft for years and nothing is done about them since unsurprisingly it take a lot of money to successfully shut down these sites, and another will pop up to replace it right away.
DMCA them if you wish but ultimately it’s a futile effort.