Is the dataset code stored in the report as text? I also find it cleaner once you start getting beyond really basic SSRS formatting to have the expression logic in a stored procedure (or even just saved in the table the report is querying). It's easier to maintain, faster to make changes etc.
For example we would make an alert column. If the value was over a certain threshold it would be green, between another threshold yellow, below dark red. We would code out that logic in SQL, so the report just had to do whatever the column told it to. A lot easier than writing SSRS expressions that are more "hidden".
It seems what you're doing is really basic though. You can easily do what you're trying to do. The expression I'd write for the font weight would be =iif(Fields!MaksPenalisasie.Value = "JA","ExtraBold","Normal"). That should work.
And yes it's possible to go crazy. If a field value is "JA" make the font bold, make the background color red, the font color white, etc. I have a lot of experience doing that.