True, there is no rule about how low you can tackle (at least not on a non-QB). In fact there is no rule about how low you can block so long as the block is from the front, not a clip or a crack.
Whether this is dirty or not depends on the intent of the player. Marty Markett is the only one who knows if this hit was dirty or not. The last time we saw this was Nik Lewis (p-tui) putting his helmet on Kelly Wiltshire's knee. Kelly never played again. He clearly deliberately put his helmet square on his knee. nik lewis is a lot bigger and taller than Wiltshire. We've all seen Lewis level guys standing straight up, he had no need to get that low. In fact it required him a lot of extra effort to get down THAT low on Wiltshire. That was dirty as hell.
Markett on Kackert, mmm, it's low, but then Markett is 5'9 and 168 lbs (according to Bombers website) one of the smallest players in the league. It would be hard for him to not get that low, although his head appeared to be lower than his butt. He is allowed to tackle low and he does appear to at least try to move his head aside and lead with shoulder pad. Kackert is a solid load so going low is a smart way to get him. It's just whether he got low properly.
It's really tough to say what his intent was on that play. The fact that his helmet seems lower than even his butt makes me think that there may have been some intent, but I just can't say. it's a serious accusation to call a player dirty. I'm not prepared to make that call on this one.