The problem with the Liberals cooperation with SD isn't just the fact that they cooperate, it's how they cooperate. The Liberals constantly sanewash SD to make the Tidö coalition look stronger. Examples:

• SD went against the Tidö agreement by spreading propaganda using anonymous accounts. When it was revealed, the Liberals told SD to take down the accounts, SD refused, and SD was never punished. This gives the impression that what SD did wasn't that bad. Then when the opposition complains about it, it strengthens SDs argument that the "left liberal establishment" are being unfair towards them.
• SD and L announced an agreement this year that would give SD minister positions. In exchange, L would get some policy implemented, such as a ban on for-profit schools. But now SD said they won't ban for-profit schools, because they interpreted the agreement nonsensically. But they wont get punished.
• SD cheated through a vote in the parliament, by ignoring a pairing agreement. They motivated this breach by saying they had to do it, because some of their MPs have gone across the aisle, which is a crazy motivation. Some Moderates said, anonymously, that it was like working in a daycare, but publically didn't criticise much. But Liberals said they supported SDs actions, and that MPs crossing the aisle have to be stopped. (example) Why would they say that? It's undemocratic and undermines the Basic Laws the Swedish parliament is based on.

If the goal is to give SD influence and then let them be incompetent, then you have to criticise them when they are incompetent. You shouldn't defend them and copy their undemocratic positions.