NYET... Be aware that Tamiya does not have a clear flat acrylic. What they do have is X-21 Flat Base, which is an additive which is used to render their gloss acrylics (including X-22 Clear Gloss) flat.
Do not attempt to use X-21 on its own without mixing with clear gloss first. Doing so will result in a translucent frosting-like effect. You can make a clear flat by mixing one part of X-21 Flat Base to four parts of X-22 Clear Gloss.
Having said that, when suitably mixed, an X-21/X-22 mix (or X-22 Clear Gloss on its own) may be used over decals without any issues.
If you are referring to Tamiya's spray cans, they are lacquers, not acrylics and may adversely affect your decals if applied too heavily.