In most cases, there is no deliberate attempt to sabotage the support act's sound or lights, but limitations might apply. Of course, depending on the venue and main act.
Some variables and possible "why's" below...:
The techs and gear. If the support act bring their own professional techs and as much gear as possible, there might be not too many negative effects on the appearance of support act, other than restricted overall volume (which might negatively affect the "impact" of the music a bit), and restrictions on visuals.
If the support act is mixed by headliner crew, don't expect them to know the support band's sound and songs by heart (the very core of a good mix), no matter how good or pro they are. It takes loads of prepping, and a lot of shows to cater to the fullest. Their no1 priority is the headliner, the one who pays them. They'll have to endure a lengthy (likely loud, too) show after the support act, so "saving their ears a bit" is understandable. The headliner gets more volume, to have most impact on the audience. It's their show, support is there to "warm up the audience".
Setup. Headliner is not likely to move their stuff onstage, once it's set up. Why? They normally dont like doing (possibly very) complicated setups twice, if it can be avoided somehow. Anything changing is likely to affect THEIR (sound and/or lighting) setup negatively, and multiplies the chances of something going wrong. Thus, the support has to adapt, to the point of having extremely limited space and a less-than-optimal setup. Also, if support does not bring their own mixing desks, mics etc, the headliner is not going to compromise their show by sharing theirs or doing massive re-patching (which might go wrong during a hurried changeover). Again, it's their show, and people are paying to see the headliner's best show.
Lighting. Say, the theme of headliner tour is red and yellow (repeated on album covers, merch etc, too). If they did not restrict the use of said colors on support act lighting, their show wouldn't make much of an impact if support act saturated their show with the same colors. I've seen support acts go with only white light. Looks bland, but makes the headliner's set really "explode". Same goes with strobes and pyros, you can only take so much in one night.
Also, there is a possibility that the headliner simply sounds better. Better arrangements, tighter playing, more thought-out sounds, carefully selected mics on each source, etc. No sound engineer can make a mediocre band sound beyond their abilities. A good mix compliments good music.