Yes that's true, she is getting worse all the time, but still she has a lot of experience. Thomsen is very inconsistent, she can make great saves but also so simple mistakes and her reception is not as it should be for a libero. In terms of libero, Vilsbiburg with Lenka Dürr and I think also Münster with Linda Dörendahl have better players, also Evelyn Delogu from Stuttgart is a very good libero.
Dürr is indeed better; she is best German libero in my opinion. The other 2 I don't know well, but considering the system Münster plays succesfully, Dörendahl must 
be a good libero
myself 
Did she never play for German NT?
It depends on the set. In the middle, when the opponent MB jumps fully and the ball is close to the net, the middle attacker has almost no chance to pass against a good blocker. On position 2, the blocker often is in lateral movement so it's very difficult to position the block well, also there the attacker can let the ball fly past the block and look where she has to attack, in the middle there is no time for that usually. There are very few players who are really good at blocking slide attacks (for example Osmokrovic), it's a matter of experience and resisting the temptation to run outside together with the attacker instead of simply standing and jumping and block the cross-court, so the attacker is forced to attack longline and there is always the risk to attack out. Of course it all depends on the height of the attacker and the blocker, when there is a 1.95 MB and a 1.80 OH in the opponent team it is easier to attack from pos 2 for the MB, but when the MB has to attack against a 1.95 OH it is equally difficult.