That's a big 'maybe.' First thing would be to find out what they are tensioned to now. This entails the use of a spoke tension-meter. Any good bike-shop that works on wheels will have one. Ask them to check this. If they are within proper tension, the answer to your perplexing flexing would be not really. If they are on the loose side - let's say 90Kgf - then they can be tightened-up a bit more.
Many rims are known to flex. And this is perfectly normal for them. Try to find out as much as possible about the rims you have. See what people have to say about riding them. If it's normal to have them flex - you can either accept this, or look for a new set of wheels with rims that don't flex as much.